In plain words
Having gained this, one considers no other gain greater; established in it, one is not shaken even by great sorrow.
Word for word
And having gained which, [one] does not think another gain greater than that; established in which, [one] is not shaken even by heavy sorrow,
The commentators
Bhagavadgītābhāṣyam
किञ्च —
यं लब्ध्वा यम् आत्मलाभं लब्ध्वा प्राप्य च अपरम् अन्यत् लाभं लाभान्तरं ततः अधिकम् अस्तीति न मन्यते न चिन्तयति । किञ्च, यस्मिन् आत्मतत्त्वे स्थितः दुःखेन शस्त्रनिपातादिलक्षणेन गुरुणा महता अपि न विचाल्यते ॥ २२ ॥
In plain words
And there is more —
"Having gained which" — that is: once you have gained, once you have actually reached, the Self. And "other" simply means other: he does not reckon, does not even entertain the thought, that some further gain exists, some different prize higher than this one. And more: standing in that reality of the Self, he is not shaken by pain — pain of the kind a blade coming down brings, and not even by the heavy kind, meaning the very greatest.
Close to the Sanskrit
And what more —
"Having gained which" — which, that is, having gained, having reached, the attainment of the Self; and "aparam" means "other": he does not think, does not consider, that there exists another gain, a gain of some other sort, greater than that. Moreover, "in which" — established in which reality of the Self, he is not shaken by pain, pain characterized by the fall of a weapon and the like, even heavy, that is, great.