How one who has attained perfection attains Brahman — know that from Me, oh Kuntīya, in summary: the firmness of knowledge which is supreme.

Kṛṣṇa summarizes the path to Brahman. Siddhiṃ prāptaḥ — one who has attained perfection. Yathā brahma āpnoti — how one attains Brahman.

Samāsena — in summary, briefly. Niṣṭhā jñānasya yā parā — the supreme firmness/stability of knowledge. Jñāna-niṣṭha leads to brahma-prāpti.