In plain words
But one who knows the tattva of the divisions of guṇa and action, mighty-armed one, understanding that the guṇas operate among the guṇas, does not become attached.
Word for word
But the knower of the truth about the divisions of guṇa and action, O mighty-armed one, having understood 'the guṇas move among the guṇas', is not attached.
The commentators
Bhagavadgītābhāṣyam
यः पुनर्विद्वान् —
तत्त्ववित् तु महाबाहो । कस्य तत्त्ववित् ? गुणकर्मविभागयोः गुणविभागस्य कर्मविभागस्य च तत्त्ववित् इत्यर्थः । गुणाः करणात्मकाः गुणेषु विषयात्मकेषु वर्तन्ते न आत्मा इति मत्वा न सज्जते सक्तिं न करोति ॥ २८ ॥
In plain words
But the one who knows —
But you, mighty-armed Arjuna, the one who knows the truth — knows the truth of what, exactly? Of how the guṇas divide from each other, and how the actions arising from them divide too. It's simply this: the senses, which are guṇas, are moving among sense-objects, which are also guṇas — one part of nature meeting another part of nature. The Self was never in that transaction at all. Once someone sees this clearly, nothing sticks to him. He doesn't get entangled — not because he's trying not to, but because there was never anything there to get caught on.
Close to the Sanskrit
But he who is a knower —
But O mighty-armed one, the knower of the truth — of what is he a knower? Of the division of the guṇas and of actions — of the division of the guṇas and of the division of actions: that is the meaning. The guṇas, consisting in the sense-organs, operate among the guṇas consisting in the sense-objects; the Self does not [operate in this] — having considered thus, he does not cling, does not produce attachment.