In plain words
Indeed, Janaka and others set out to reach perfection through action alone. You should act also with a view to lokasaṅgraha—keeping the world from going astray.
Word for word
For by action alone Janaka and others set out for perfection; looking also to the holding together of the world, you ought to act.
The commentators
Bhagavadgītābhāṣyam
यस्माच्च —
कर्मणैव हि यस्मात् पूर्वे क्षत्रियाः विद्वांसः संसिद्धिं मोक्षं गन्तुम् आस्थिताः प्रवृत्ताः । के ? जनकादयः जनकाश्वपतिप्रभृतयः । यदि ते प्राप्तसम्यग्दर्शनाः, ततः लोकसङ्ग्रहार्थं प्रारब्धकर्मत्वात् कर्मणा सहैव असंन्यस्यैव कर्म संसिद्धिमास्थिता इत्यर्थः । अथ अप्राप्तसम्यग्दर्शनाः जनकादयः, तदा कर्मणा सत्त्वशुद्धिसाधनभूतेन क्रमेण संसिद्धिमास्थिता इति व्याख्येयः श्लोकः । अथ मन्यसे पूर्वैरपि जनकादिभिः अजानद्भिरेव कर्तव्यं कर्म कृतम् ; तावता नावश्यमन्येन कर्तव्यं सम्यग्दर्शनवता कृतार्थेनेति ; तथापि प्रारब्धकर्मायत्तः त्वं लोकसङ्ग्रहम् एव अपि लोकस्य उन्मार्गप्रवृत्तिनिवारणं लोकसङ्ग्रहः तमेवापि प्रयोजनं सम्पश्यन् कर्तुम् अर्हसि ॥ २० ॥
In plain words
And because —
It was through action itself, remember, that the learned kings of old — Janaka, Aśvapati, and others like them — stood firm and reached perfection, reached liberation. Maybe they had already seen the truth fully, and kept acting anyway, for the world's sake, and because a king's work, once begun, doesn't simply stop. Or maybe they hadn't yet seen it fully, and action was exactly what cleared and settled their minds until they did, step by step. Either reading works. Now you might say to yourself: 'Fine, but those ancient kings acted without full knowledge — that's why they had to. Someone who already sees the truth, whose work is already done inside, isn't bound by the same need.' True enough. And still — your own work has already begun, Arjuna, and cannot simply be dropped. So act anyway, with this one purpose clearly in view: holding the world together, keeping people from wandering off the path they should be walking. That alone is reason enough.
Close to the Sanskrit
And because —
For, indeed, since it was by action alone that the ancient kshatriya kings, who were learned, stood engaged, set forth, to attain perfection, that is, liberation — who were they? Janaka and the others, Janaka, Aśvapati, and the like. If they had already attained right vision, then, for the sake of the world's welfare and because their action was already underway, they remained standing in perfection together with action, without renouncing action — that is the meaning. Or, if Janaka and the others had not yet attained right vision, then by action, which served as a means of purifying the mind, they became established in perfection by stages — so should the verse be explained. Now you may think: even by the ancients, by Janaka and the others, who acted without full knowledge, prescribed duty was performed; on that ground alone it does not necessarily follow that another person, one who possesses right vision and whose purpose is already accomplished, must also act. Even so, being bound to action already commenced, you should perform action keeping in view precisely this purpose too — the world's welfare, that is, the prevention of people's proceeding onto a wrong path.