In plain words
When light, which is jñāna, arises through all the gates of this body, then one should know that sattva has increased.
Word for word
When in all the gates in this body light arises, [that is] jñāna, then one should know that sattva has grown indeed.
सर्वद्वारेषु देहेऽस्मिन् प्रकाश उपजायतेज्ञानं यदा तदा विद्याद् विवृद्धं सत्त्वमित्युत
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
यदा यो गुणः उद्भूतः भवति, तदा तस्य किं लिङ्गमिति उच्यते —
सर्वद्वारेषु, आत्मनः उपलब्धिद्वाराणि श्रोत्रादीनि सर्वाणि करणानि, तेषु सर्वद्वारेषु अन्तःकरणस्य बुद्धेः वृत्तिः प्रकाशः देहे अस्मिन् उपजायते । तदेव ज्ञानम् । यदा एवं प्रकाशो ज्ञानाख्यः उपजायते, तदा ज्ञानप्रकाशेन लिङ्गेन विद्यात् विवृद्धम् उद्भूतं सत्त्वम् इति उत अपि ॥ ११ ॥
In plain words
When one of the guṇas rises up and takes over, what is the sign of it? That is what is said next —
'In all the gates' means the senses through which the self takes things in: the ear and the rest, every one of those instruments. It's among these gates, in the functioning of the mind's inner instrument, the intellect, that a kind of light arises within this body. That light is exactly what knowledge is. And when this light we call knowledge shows up like this, take it as a sign: sattva has grown, sattva has come forward. That's what's meant.
Close to the Sanskrit
When whichever guṇa becomes risen, then what is its mark — thus it is said —
'In all the gates' [means]: the gates of perception belonging to the self — the ear and so on, all the organs; among those, in all the gates, a functioning of the inner instrument, of the intellect, [namely] manifestation, arises in this body. That indeed is knowledge. When thus a manifestation named knowledge arises, then by that manifestation of knowledge as a sign one should know that sattva has increased, has become manifest — so it is said.