Gita Mahatmya
Śrī Śaṅkarācārya, Gītā-bhāṣya, Introduction
These verses glorify the Bhagavad-gītā. They come from other scriptures and are not verses of the Bhagavad-gītā itself. Each one carries its own source.
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Gītā-māhātmya
तद् इदं गीताशास्त्रं समस्तवेदार्थसारसंग्रहभूतं दुर्विज्ञेयार्थम् ।
tad idaṁ gītāśāstraṁ samastavedārthasārasaṁgrahabhūtaṁ durvijñeyārtham
This Gītā-śāstra holds the essence of the meaning of all the Vedas. Its meaning is not easy to understand.
Source: Śrī Śaṅkarācārya, Gītā-bhāṣya, Introduction