ऋषिभिर्बहुधा गीतं छन्दोभिर्विविधैः पृथक्। ब्रह्मसूत्रपदैश्चैव हेतुमद्भिर्विनिश्िचतैः ॥

ṛiṣhibhir bahudhā gītaṁ chhandobhir vividhaiḥ pṛithak brahma-sūtra-padaiśh chaiva hetumadbhir viniśhchitaiḥ

Rishis have sung in manifold ways, in many various chants, and in decisive Brahma-sûtra verses full of reasonings.

In plain words: Krishna notes that this truth has been sung by Rishis, ancient wise seers, in many different hymns. It has also been reasoned out in the Brahma-sutras, short sacred verses about the ultimate reality.