In plain words
who sits as though unconcerned and is not shaken by the guṇas, remaining steady in the understanding, ‘The guṇas alone are functioning,’ and not wavering,
Word for word
Who, sitting like one unconcerned, is not moved by the guṇas; who, [knowing] 'the guṇas alone act', stands firm and does not stir,
उदासीनवदासीनो गुणैर्यो न विचाल्यतेगुणा वर्तन्त इत्येव योऽवतिष्ठति नेङ्गते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
अथ इदानीम् ‘गुणातीतः किमाचारः ? ’ इति प्रश्नस्य प्रतिवचनम् आह —
उदासीनवत् यथा उदासीनः न कस्यचित् पक्षं भजते, तथा अयं गुणातीतत्वोपायमार्गेऽवस्थितः आसीनः आत्मवित् गुणैः यः संन्यासी न विचाल्यते विवेकदर्शनावस्थातः । तदेतत् स्फुटीकरोति — गुणाः कार्यकरणविषयाकारपरिणताः अन्योऽन्यस्मिन् वर्तन्ते इति यः अवतिष्ठति । छन्दोभङ्गभयात् परस्मैपदप्रयोगः । योऽनुतिष्ठतीति वा पाठान्तरम् । न इङ्गते न चलति, स्वरूपावस्थ एव भवति इत्यर्थः ॥ २३ ॥
In plain words
Now Kṛṣṇa answers the question that was just asked: what does someone who has moved beyond the guṇas actually look like? —
'Like one who stays neutral': just as a neutral person takes no one's side, this person too stands firm — still walking the path toward full transcendence of the guṇas, settled, a knower of the Self, a renunciate whom the guṇas cannot shake, never falling out of that state of clear discernment. The verse itself spells this out further: 'the guṇas, taking shape as every effect, instrument, and object, simply act on one another' — and the one who stays steady through all of that is exactly who is meant. (The slightly unusual grammar here is only to preserve the metre; some texts read this word a little differently, and that's a known, acceptable variant.) He doesn't stir, he doesn't waver — he simply rests in what he already is. That's the whole of it.
Close to the Sanskrit
Now, [Kṛṣṇa] states the answer to the question 'Of what conduct is one who has transcended the guṇas?' —
'Like one who is indifferent': just as an indifferent person favors no one's side, so this one, established on the path that is the means to transcending the guṇas, seated, a knower of the Self, who as a renunciate is not shaken by the guṇas, [does not fall] from his state of discerning vision. This very point the verse makes clear: 'the guṇas, transformed into the forms of effect, instrument and object, operate among one another' — thus [reflecting], he who remains established [is meant]. Out of fear of breaking the metre, [there is] the use of the parasmaipada ending. 'Yo'nutiṣṭhati' — or, this is a variant reading. He does not stir, does not waver; he simply abides in his own nature: this is the meaning.