In plain words
Thus this most secret teaching has been declared by Me, sinless one. Knowing it, one becomes endowed with buddhi and has fulfilled what is to be fulfilled, Bharata.
Word for word
Thus this most secret śāstra has been told by Me, O sinless one; having understood this, one would be wise and [one] whose task is done, O Bhārata.
इति गुह्यतमं शास्त्रमिदमुक्तं मयाऽनघएतद् बुद्ध्वा बुद्धिमान् स्यात् कृतकृत्यश्च भारत
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
अस्मिन् अध्याये भगवत्तत्त्वज्ञानं मोक्षफलम् उक्त्वा,अथ इदानीं तत् स्तौति —
इति एतत् गुह्यतमं गोप्यतमम् , अत्यन्तरहस्यं इत्येतत् । किं तत् ? शास्त्रम् । यद्यपि गीताख्यं समस्तम् ‘शास्त्रम्’ उच्यते, तथापि अयमेव अध्यायः इह ‘शास्त्रम्’ इति उच्यते स्तुत्यर्थं प्रकरणात् । सर्वो हि गीताशास्त्रार्थः अस्मिन् अध्याये समासेन उक्तः । न केवलं गीताशास्त्रार्थ एव, किन्तु सर्वश्च वेदार्थः इह परिसमाप्तः । ‘यस्तं वेद स वेदवित्’(भ. गी. १५ । १) ‘वेदैश्च सर्वैरहमेव वेद्यः’(भ. गी. १५ । १५) इति च उक्तम् । इदम् उक्तं कथितं मया हे अनघ अपाप । एतत् शास्त्रं यथादर्शितार्थं बुद्ध्वा बुद्धिमान् स्यात् भवेत् न अन्यथा कृतकृत्यश्च भारत कृतं कृत्यं कर्तव्यं येन सः कृतकृत्यः ; विशिष्टजन्मप्रसूतेन ब्राह्मणेन यत् कर्तव्यं तत् सर्वं भगवत्तत्त्वे विदिते कृतं भवेत् इत्यर्थः ; न च अन्यथा कर्तव्यं परिसमाप्यते कस्यचित् इत्यभिप्रायः । ‘सर्वं कर्माखिलं पार्थ ज्ञाने परिसमाप्यते’(भ. गी. ४ । ३३) इति च उक्तम् । ‘एतद्धि जन्मसामग्र्यं ब्राह्मणस्य विशेषतः । प्राप्यैतत्कृतकृत्यो हि द्विजो भवति नान्यथा’(मनु. १२ । ९३) इति च मानवं वचनम् । यतः एतत् परमार्थतत्त्वं मत्तः श्रुतवान् असि, अतः कृतार्थः त्वं भारत इति ॥ २० ॥
In plain words
In this chapter, having taught the knowledge of who the Lord truly is, a knowledge whose fruit is liberation itself, he now turns to praise that very knowledge —
'Thus,' this teaching is the most secret, the most hidden, utterly mysterious. What is it? It is scripture itself. Although the whole text we call the Gītā is generally called 'scripture,' even so, this one chapter alone is called 'scripture' here, in praise, because of what it contains: the entire meaning of the Gītā has, in fact, been condensed into this single chapter. And not only the meaning of the Gītā, but the whole meaning of the Veda itself finds its completion here. 'Whoever knows this is a knower of the Veda' (Bhagavad Gītā 15.1), and 'by all the Vedas, I alone am to be known' (Bhagavad Gītā 15.15), so it has already been said. This teaching I have now given you, spoken to you, Arjuna, sinless one, faultless one. Understanding this scripture exactly as it has been shown to you, you will become wise, not otherwise, and your task will be accomplished. To have accomplished one's task means to have done the duty that was one's to do; whatever duty belongs to a brahmin by virtue of his distinguished birth, all of it counts as done, once the truth of the Lord is truly known, that is what it means; and without that knowledge, no one's duty is ever really completed, that is the point. 'All action, everything, Pārtha, culminates in knowledge,' this too has been said. And Manu himself says: 'This indeed is what makes a birth complete, especially for a brahmin; having attained this, the twice-born truly accomplishes his task, and not otherwise.' Because you have heard this highest truth from Me, you have now accomplished your purpose, Arjuna.
Close to the Sanskrit
In this chapter, having taught the knowledge of the reality of the Lord, which has liberation as its fruit, now, at this point, he praises it —
'Thus' — this, the most secret, the most hidden, the utterly mysterious — this. What is that? The scripture. Although the entire text called the Gītā is called 'śāstra,' nevertheless this very chapter here is called 'śāstra,' for the sake of praise, on account of the context. For indeed the whole meaning of the Gītā-scripture has been stated, in brief, in this chapter. Not only the meaning of the Gītā-scripture alone, but the entire meaning of the Veda too is completed here. 'He who knows that, he is a knower of the Veda' (Bhagavad Gītā 15.1), 'by all the Vedas, I alone am to be known' (Bhagavad Gītā 15.15), thus has it also been said. This has been said, declared, by Me, O sinless one, O faultless one. Having understood this scripture, as its meaning has been shown, one would become wise, not otherwise, and one whose task is accomplished, O Bhārata; 'done' is the duty that ought to be done, by whom that is done, he is 'one whose task is accomplished'; whatever ought to be done by a brahmin born of a distinguished birth, all of that would be done, when the truth of the Lord is known; that is the meaning; and otherwise, duty is not completed for anyone; this is the purport. 'All action, entire, O Pārtha, is completed in knowledge' (Bhagavad Gītā 4.33), thus has it also been said. 'For this indeed is the fulfillment of birth for a brahmin especially; having attained this, the twice-born indeed becomes one whose task is accomplished, not otherwise' (Manu 12.93), thus is also the statement of Manu. Because you have heard this supreme truth from Me, therefore you are one whose purpose is accomplished, O Bhārata. || 20 ||