In plain words
Know that what is called Sannyāsa is indeed yoga, Pandava; no one becomes a yogin without renouncing saṅkalpa.
Word for word
What they call saṃnyāsa, know that [to be] yoga, O son of Pāṇḍu; for no one who has not renounced saṅkalpa becomes a yogin.
The commentators
Bhagavadgītābhāṣyam
ननु च निरग्नेः अक्रियस्यैव श्रुतिस्मृतियोगशास्त्रेषु संन्यासित्वं योगित्वं च प्रसिद्धम् । कथम् इह साग्नेः सक्रियस्य च संन्यासित्वं योगित्वं च अप्रसिद्धमुच्यते इति । नैष दोषः, कयाचित् गुणवृत्त्या उभयस्य सम्पिपादयिषितत्वात् । तत् कथम् ? कर्मफलसङ्कल्पसंन्यासात् संन्यासित्वम् , योगाङ्गत्वेन च कर्मानुष्ठानात् कर्मफलसङ्कल्पस्य च चित्तविक्षेपहेतोः परित्यागात् योगित्वं च इति गौणमुभयम् ; न पुनः मुख्यं संन्यासित्वं योगित्वं च अभिप्रेतमित्येतमर्थं दर्शयितुमाह —
यं सर्वकर्मतत्फलपरित्यागलक्षणं परमार्थसंन्यासं संन्यासम् इति प्राहुः श्रुतिस्मृतिविदः, योगं कर्मानुष्ठानलक्षणं तं परमार्थसंन्यासं विद्धि जानीहि हे पाण्डव । कर्मयोगस्य प्रवृत्तिलक्षणस्य तद्विपरीतेन निवृत्तिलक्षणेन परमार्थसंन्यासेन कीदृशं सामान्यमङ्गीकृत्य तद्भाव उच्यते इत्यपेक्षायाम् इदमुच्यते — अस्ति हि परमार्थसंन्यासेन सादृश्यं कर्तृद्वारकं कर्मयोगस्य । यो हि परमार्थसंन्यासी स त्यक्तसर्वकर्मसाधनतया सर्वकर्मतत्फलविषयं सङ्कल्पं प्रवृत्तिहेतुकामकारणं संन्यस्यति । अयमपि कर्मयोगी कर्म कुर्वाण एव फलविषयं सङ्कल्पं संन्यस्यति इत्येतमर्थं दर्शयिष्यन् आह — न हि यस्मात् असंन्यस्तसङ्कल्पः असंन्यस्तः अपरित्यक्तः फलविषयः सङ्कल्पः अभिसन्धिः येन सः असंन्यस्तसङ्कल्पः कश्चन कश्चिदपि कर्मी योगी समाधानवान् भवति न सम्भवतीत्यर्थः, फलसङ्कल्पस्य चित्तविक्षेपहेतुत्वात् । तस्मात् यः कश्चन कर्मी संन्यस्तफलसङ्कल्पो भवेत् स योगी समाधानवान् अविक्षिप्तचित्तो भवेत् , चित्तविक्षेपहेतोः फलसङ्कल्पस्य संन्यस्तत्वादित्यभिप्रायः ॥ २ ॥
In plain words
But surely — scripture, tradition, and the yoga treatises all agree that only someone with no ritual fire and no ritual action truly earns the titles renunciant and yogin. So how can these same titles now be applied here to someone who still has ritual fire and still performs ritual action, even though that's not the established meaning? This isn't actually a problem, because both titles are being used here in a secondary, extended sense. How so? Renunciant-status comes simply from giving up the intention for the fruit of action; and yogin-status comes, as a support for yoga, from performing action while dropping that same fruit-seeking intention, which is exactly what scatters the mind. So both titles here are being used loosely, not in their strict, primary sense. To make this clear, Krishna says—
'Which' is characterized by giving up every action and its fruit — that transcendent renunciation is what scripture and tradition call 'renunciation.' 'Yoga' is characterized by the actual performance of action — 'know that,' says Krishna, that very transcendent renunciation, son of Pāṇḍu. Naturally one might ask: what shared ground makes it right to equate the yoga of action, which means engaging, with transcendent renunciation, which means withdrawing — its exact opposite? Here's the answer: they do resemble each other, through the agent. A true transcendent renunciant, having given up every means of acting, gives up the very intention behind all action and its fruit — the desire that drives him to act at all. And the practitioner of the yoga of action does something similar: even while performing action, he gives up the intention for its fruit. To make this point, Krishna says: because anyone whose intention isn't renounced — whose resolve about the fruit still stands, unrenounced, unabandoned — such a person, no matter who, engaged in action, simply cannot become a composed yogin. That's the meaning, because the intention for the fruit is exactly what scatters the mind. So whoever, engaged in action, has renounced that intention for the fruit becomes a yogin, composed, undistracted in mind — precisely because the intention for the fruit, the very cause of the mind's scattering, has been let go. That's the point.
Close to the Sanskrit
But surely, in scripture, tradition, and the yoga treatises, renunciant-status and yogin-status are well-established only for one without ritual fire and without ritual action; how, here, is renunciant-status and yogin-status spoken of as [also applying, though] not established, for one who has ritual fire and ritual action? This is not a flaw, since both are intended to be established by a certain secondary usage. How is that? Renunciant-status comes from the renunciation of the intention for the fruit of action, and, as an accessory of yoga, yogin-status comes from performing action and from abandoning the intention for the fruit of action, which is the cause of mental distraction — thus both are secondary [uses of the terms]; the primary [sense of] renunciant-status and yogin-status is not, however, intended here. To show this point, he says—
'Which,' characterized by the abandonment of all action and its fruit, [i.e.] transcendent renunciation, 'renunciation' — those who know scripture and tradition call it; 'yoga,' characterized by the performance of action — 'know that' [very] transcendent renunciation, [i.e.] know, O son of Pāṇḍu. When it is asked, accepting what kind of commonality with the yoga of action, characterized by engagement, is that identity asserted with its opposite, transcendent renunciation, characterized by withdrawal — this is the answer: there is, indeed, a resemblance, by way of the agent, that the yoga of action has with transcendent renunciation. For whoever is a transcendent renunciant, he, by virtue of having abandoned every means of action, renounces the intention concerning all action and its fruit, which is the desire-born cause of engagement. This practitioner of the yoga of action, too, while performing action, renounces the intention concerning the fruit — about to show this point, he says: 'for, since one whose intention is not renounced' — [i.e.] not renounced, not abandoned, the intention, the resolve, concerning the fruit, by whom [it is not renounced] — he, 'one whose intention is not renounced,' any, anyone at all who is engaged in action, 'becomes a yogin, possessed of composure' — is not possible. That is the meaning, since the intention for the fruit is itself the cause of mental distraction. Therefore, whoever, any person engaged in action, has renounced the intention for the fruit, he is a yogin, possessed of composure, [i.e.] of undistracted mind, would he be — since the intention for the fruit, which is the cause of mental distraction, is renounced. That is the purport.