In plain words
Mentally renouncing all actions, the self-controlled embodied one rests happily in the city of nine gates, neither acting nor causing action.
Word for word
Having renounced all actions with the mind, the self-controlled embodied one abides happily in the city of nine gates, neither doing at all nor causing [others] to act.
The commentators
Bhagavadgītābhāṣyam
यस्तु परमार्थदर्शी सः —
सर्वाणि कर्माणि सर्वकर्माणि संन्यस्य परित्यज्य नित्यं नैमित्तिकं काम्यं प्रतिषिद्धं च तानि सर्वाणि कर्माणि मनसा विवेकबुद्ध्या, कर्मादौ अकर्मसन्दर्शनेन सन्त्यज्येत्यर्थः, आस्ते तिष्ठति सुखम् । त्यक्तवाङ्मनःकायचेष्टः निरायासः प्रसन्नचित्तः आत्मनः अन्यत्र निवृत्तसर्वबाह्यप्रयोजनः इति ‘सुखम् आस्ते’ इत्युच्यते । वशी जितेन्द्रिय इत्यर्थः । क्व कथम् आस्ते इति, आह — नवद्वारे पुरे । सप्त शीर्षण्यानि आत्मन उपलब्धिद्वाराणि, अवाक् द्वे मूत्रपुरीषविसर्गार्थे, तैः द्वारैः नवद्वारं पुरम् उच्यते शरीरम् , पुरमिव पुरम् , आत्मैकस्वामिकम् , तदर्थप्रयोजनैश्च इन्द्रियमनोबुद्धिविषयैः अनेकफलविज्ञानस्य उत्पादकैः पौरैरिव अधिष्ठितम् । तस्मिन् नवद्वारे पुरे देही सर्वं कर्म संन्यस्य आस्ते ; किं विशेषणेन ? सर्वो हि देही संन्यासी असंन्यासी वा देहे एव आस्ते ; तत्र अनर्थकं विशेषणमिति । उच्यते — यस्तु अज्ञः देही देहेन्द्रियसङ्घातमात्रात्मदर्शी स सर्वोऽपि ‘गेहे भूमौ आसने वा आसे’ इति मन्यते । न हि देहमात्रात्मदर्शिनः गेहे इव देहे आसे इति प्रत्ययः सम्भवति । देहादिसङ्घातव्यतिरिक्तात्मदर्शिनस्तु ‘देहे आसे’ इति प्रत्ययः उपपद्यते । परकर्मणां च परस्मिन् आत्मनि अविद्यया अध्यारोपितानां विद्यया विवेकज्ञानेन मनसा संन्यास उपपद्यते । उत्पन्नविवेकज्ञानस्य सर्वकर्मसंन्यासिनोऽपि गेहे इव देहे एव नवद्वारे पुरे आसनम् प्रारब्धफलकर्मसंस्कारशेषानुवृत्त्या देह एव विशेषविज्ञानोत्पत्तेः । देहे एव आस्ते इति अस्त्येव विशेषणफलम् , विद्वदविद्वत्प्रत्ययभेदापेक्षत्वात् ॥
यद्यपि कार्यकरणकर्माणि अविद्यया आत्मनि अध्यारोपितानि ‘संन्यस्यास्ते’ इत्युक्तम् , तथापि आत्मसमवायि तु कर्तृत्वं कारयितृत्वं च स्यात् इति आशङ्क्य आह — नैव कुर्वन् स्वयम् , न च कार्यकरणानि कारयन् क्रियासु प्रवर्तयन् । किं यत् तत् कर्तृत्वं कारयितृत्वं च देहिनः स्वात्मसमवायि सत् संन्यासात् न सम्भवति, यथा गच्छतो गतिः गमनव्यापारपरित्यागे न स्यात् तद्वत् ? किं वा स्वत एव आत्मनः न अस्ति इति ? अत्र उच्यते — न अस्ति आत्मनः स्वतः कर्तृत्वं कारयितृत्वं च । उक्तं हि ‘अविकार्योऽयमुच्यते’(भ. गी. २ । २५) ‘शरीरस्थोऽपि न करोति न लिप्यते’(भ. गी. १३ । ३१) इति । ‘ध्यायतीव लेलायतीव’(बृ. उ. ४ । ३ । ७) इति श्रुतेः ॥ १३ ॥
In plain words
But the one who sees the highest truth — he —
'All actions' — every single one — 'having renounced': having let go of the daily, the occasional, the optional, and the forbidden alike, all of them, with the mind, with a discriminating intellect that sees non-action at the very root of action. 'He sits happily': having dropped every activity of speech, mind, and body, unstrained, tranquil, with no purpose left outside the Self itself — that's why it's called sitting happily. 'Self-controlled' means his senses are mastered. Where does he sit, and how? In the nine-gated city, Krishna says. Seven gates sit in the head — the doors through which the self perceives; two more, below, exist for excreting waste. Through these nine gates, the body earns the name 'city' — a city with the Self as its one and only ruler, populated, like citizens, by the senses, mind, intellect, and their objects, all serving that ruler's purposes and producing endless experience. In that nine-gated city, the embodied self, having renounced all action, sits. Why add 'in the nine-gated city' at all — doesn't every embodied being, renounced or not, simply sit in the body? Wouldn't that make the phrase pointless? Here's the answer: an ignorant person, who takes the mere heap of body and senses to be the self, thinks, indifferently, 'I'm sitting in the house,' 'on the ground,' or 'on a chair' — body, house, ground, chair, all the same kind of location to him. Someone who sees only the body as self could never really say 'I sit in the body' the way he says 'I sit in the house' — that thought doesn't even arise for him. But for someone who sees the Self as entirely distinct from the body and its aggregate, the thought 'I sit in the body' makes perfect sense — because only he can recognize that the body's actions belong to something other than himself, mistakenly projected onto the true Self by ignorance, and let them go through knowledge, through discriminative wisdom, with the mind. And even someone who has this discriminative knowledge, who has renounced everything, still goes on sitting in the body — the way one goes on sitting in a house — in this nine-gated city, because the momentum of already-ripening past action keeps producing the specific experience of being embodied. So yes, 'he sits only in the body' really does carry a purpose — it marks exactly the difference between how a knower experiences this and how an ignorant person does.
Although the doings of the body's own instruments only ever get pinned onto the Self by ignorance — and even though we've already said, 'he renounces them and sits' — someone might still wonder: doesn't some real agency, some capacity to act or to cause action, still belong to the Self itself? Anticipating exactly that doubt, Krishna says: not acting at all, himself, nor causing the organs to act, nor driving them into their activities. So which is it — does agency belong to the embodied self's own nature and simply fail to stop with renunciation, the way a walking man's momentum doesn't vanish the instant he decides to stop walking? Or does the Self never have any such agency to begin with? Here's the answer: the Self never had any agency, any capacity to cause action, of its own, at all. As it's already been said: 'this Self is declared changeless'; 'though it dwells in the body, it does not act, it is not touched.' And scripture confirms it: 'it only seems to think, only seems to stir.'
Close to the Sanskrit
But he who is a seer of the highest reality, he —
'All actions' — all actions — 'having renounced,' having relinquished the obligatory, the occasional, the optional, and the prohibited, all those actions, 'with the mind,' [i.e.] with the discriminating intellect, by seeing non-action at the very root of action — that is the meaning of 'wholly abandons.' 'He sits,' remains, 'happily' — having abandoned all activity of speech, mind, and body, without exertion, of serene mind, one for whom every external purpose apart from the Self has ceased — for this reason it is said, 'he sits happily.' 'The self-controlled one' means one whose senses are conquered. That is the meaning. Where, or how, does he sit? He says: 'in the nine-gated city.' Seven [gates] are situated in the head, the gates of perception of the self; two are downward, for excreting urine and feces; by those gates the body is called the 'nine-gated city' — a city, like a city — having the Self alone as its sole master, and inhabited, like citizens, by the senses, mind, intellect, and their objects, which have that [Self's] purpose as their aim and are the producers of manifold experience of results. In that nine-gated city, 'the embodied self, having renounced all action, sits.' What use is this qualification? For every embodied being, whether renunciant or not, sits only in the body; in that case the qualification would be pointless — [so the objection runs]. It is answered: whoever, being ignorant, an embodied being who regards the mere aggregate of body and senses as the self — every such person thinks, 'I sit in the house, on the ground, or on a seat.' For one who regards the body alone as the self, the notion 'I sit in the body,' as [one says] 'in the house,' is not possible. But for one who regards the self as distinct from the body-aggregate, the notion 'I sit in the body' is tenable. And the renunciation, with the mind, by discriminative knowledge, of actions belonging to another [the body], superimposed upon the alien self by ignorance, becomes tenable [only for such a one]. For even the renunciant of all action in whom discriminative knowledge has arisen still has his sitting, as in a house, in the body, in the nine-gated city — because, through the continuation of the residual impressions of already-commenced [prārabdha] action, a specific cognition [of embodiment] arises precisely in the body. Thus, 'he sits only in the body' — there is indeed a purpose to the qualification, since it depends on the difference between the cognition of the knower and of the non-knower.
Although the actions of the body's instruments are superimposed upon the Self by ignorance, and it was said, 'having renounced [them], he sits' — nevertheless, [one might suppose that] agency and causal agency, inherent in the Self, might still remain. Anticipating this doubt, he says: 'not at all acting himself, nor causing the organs to act, nor impelling [them] in their activities.' Is it that agency and causal agency, though inherent in the embodied self's own self, does not cease by renunciation — just as the motion of one who is walking does not cease upon abandoning the activity of walking, in the same way? Or is there, for the Self, no [such agency] at all, inherently? Here it is answered: there is, for the Self, no inherent agency or causal agency at all. For it was said, 'this [Self] is declared changeless' (Gītā 2.25); 'though abiding in the body, it does not act, is not tainted' (Gītā 13.31). [And also] 'it seems to think, as it were; it seems to tremble, as it were' (Bṛhadāraṇyaka Up. 4.3.7), by the Śruti's declaration.