In plain words
One who knows in tattva this Vibhūti and yoga of Mine becomes joined to unwavering yoga—of this there is no doubt.
Word for word
This vibhūti and yoga of Mine, who knows in truth, he is joined with unwavering yoga; there is no doubt in this.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
एतां यथोक्तां विभूतिं विस्तारं योगं च युक्तिं च आत्मनः घटनम् , अथवा योगैश्वर्यसामर्थ्यं सर्वज्ञत्वं योगजं योगः उच्यते, मम मदीयं योगं यः वेत्ति तत्त्वतः तत्त्वेन यथावदित्येतत् , सः अविकम्पेन अप्रचलितेन योगेन सम्यग्दर्शनस्थैर्यलक्षणेन युज्यते सम्बध्यते । न अत्र संशयः न अस्मिन् अर्थे संशयः अस्ति ॥ ७ ॥
In plain words
This glory of mine I have just described — call it its full expanse. And this yoga of mine — call it the very shaping of the Self itself. Or, put another way: it is the very capacity of yogic sovereignty, an omniscience born of yoga. Whoever truly knows this yoga of mine exactly as it is becomes joined, bound, to an unwavering, unshakeable yoga marked by the steadiness of right seeing. About this there is no doubt — none at all.
Close to the Sanskrit
This, as described, glory — expanse — and yoga — connection, that is, the forming of the Self; or else: the capacity of yogic sovereignty, omniscience born of yoga, is called yoga — my, that is, belonging to me, yoga, whoever knows in truth, by its very essence, this being what 'exactly as it is' means — he, by an unwavering, unshifting yoga characterized by the steadiness of right vision, is united, is bound. Not here is there doubt; not in this matter is there doubt.