In plain words
Sannyāsa is difficult to attain without yoga, mighty-armed one; the sage endowed with yoga quickly reaches Brahman.
Word for word
But renunciation, O mighty-armed one, [is] hard to attain without yoga; the muni endowed with yoga reaches brahman before long.
The commentators
Bhagavadgītābhāṣyam
एवं तर्हि योगात् संन्यास एव विशिष्यते ; कथं तर्हि इदमुक्तम् ‘तयोस्तु कर्मसंन्यासात् कर्मयोगो विशिष्यते’(भ. गी. ५ । २) इति ? शृणु तत्र कारणम् — त्वया पृष्टं केवलं कर्मसंन्यासं कर्मयोगं च अभिप्रेत्य तयोः अन्यतरः कः श्रेयान् इति । तदनुरूपं प्रतिवचनं मया उक्तं कर्मसंन्यासात् कर्मयोगः विशिष्यते इति ज्ञानम् अनपेक्ष्य । ज्ञानापेक्षस्तु संन्यासः साङ्ख्यमिति मया अभिप्रेतः । परमार्थयोगश्च स एव । यस्तु कर्मयोगः वैदिकः स च तादर्थ्यात् योगः संन्यास इति च उपचर्यते । कथं तादर्थ्यम् इति उच्यते
संन्यासस्तु पारमार्थिकः हे महाबाहो दुःखम् आप्तुं प्राप्तुम् अयोगतः योगेन विना । योगयुक्तः वैदिकेन कर्मयोगेन ईश्वरसमर्पितरूपेण फलनिरपेक्षेण युक्तः, मुनिः मननात् ईश्वरस्वरूपस्य मुनिः, ब्रह्म — परमात्मज्ञाननिष्ठालक्षणत्वात् प्रकृतः संन्यासः ब्रह्म उच्यते, ‘न्यास इति ब्रह्मा ब्रह्मा हि परः’(तै. ना. ७८) इति श्रुतेः — ब्रह्म परमार्थसंन्यासं परमार्थज्ञाननिष्ठालक्षणं नचिरेण क्षिप्रमेव अधिगच्छति प्राप्नोति । अतः मया उक्तम् ‘कर्मयोगो विशिष्यते’(भ. गी. ५ । २) इति ॥ ६ ॥
In plain words
'So then, renunciation itself must be superior to the path of action — so how, then, could you have said, "of the two, the path of action is superior to renunciation" (5.2)?' Listen to why: your question meant simply renunciation of action versus the path of action — which of the two, plainly, is better. And my answer, matching that question, was 'the path of action is superior to renunciation' — with no reference to knowledge at all. But renunciation joined with knowledge is what I meant by 'Sāṅkhya' — and that is itself the supreme yoga. The ordinary, Vedic path of action, however, since it serves that same ultimate purpose, is loosely called both 'yoga' and 'renunciation'. How does it serve that purpose? That is explained now —
'Renunciation' here means the ultimate kind, mighty-armed Arjuna — and it's hard to reach without yoga first. But someone who has yoga — who performs Vedic action offered up to the Lord, unconcerned with its fruit — such a person is a muni, so called because he reflects on the Lord's own nature. And he reaches Brahman — because this renunciation we're speaking of is nothing but steadiness in knowledge of the supreme Self, it is itself called 'Brahman,' as scripture says: 'renunciation is Brahman; Brahman alone is supreme.' He reaches Brahman — that ultimate renunciation resting on ultimate knowledge — without delay, swiftly. That's why I said earlier, 'the yoga of action is superior.'
Close to the Sanskrit
'So then, renunciation alone is superior to the path of action — then how was it said, "but of the two, the path of action is superior to renunciation" (5.2)?' Listen to the reason for that: by you, the question was asked meaning merely renunciation of action and the path of action, which of the two is better. And the reply I gave, corresponding to that, was 'the path of action is superior to renunciation of action' — without any reference to knowledge. But renunciation dependent on knowledge is what I meant by 'Sāṅkhya'; and that itself is the supreme yoga. But the path of action that is Vedic — that, because it serves that same purpose, is figuratively called both 'yoga' and 'renunciation'. How is this being-for-that-purpose so? — this is now explained —
'Renunciation,' however — [i.e.] transcendent [renunciation], O mighty-armed one — is difficult to attain, to reach, without yoga, without [karma-]yoga. One endowed with yoga, endowed with Vedic karma-yoga offered in the form dedicated to the Lord, indifferent to its fruit — [is] the muni, [so called] from reflecting on the Lord's own nature, muni — [attains] Brahman: because the renunciation under discussion is characterized by steadfastness in knowledge of the supreme Self, it is called 'Brahman,' by the Śruti's declaration, 'renunciation (nyāsa) is called Brahman; Brahman indeed is supreme' (Tai. Nā. 78) — [he] attains, reaches, Brahman, [i.e.] transcendent renunciation characterized by steadfastness in transcendent knowledge, without delay, quickly. Therefore I have said, 'karma-yoga is superior' (Gītā 5.2).