Is there any text prior to sankara bhashya which mention about it?
Yes ,Shandilya Bhakti Sutras mentions Shreemad Bhagvat Gita . Rishi Shandilya is said to be the author of Shandilya Bhakti Sutras during the time period of the Mahabharata (300 BCE. ). However the exact date of the text is a point of debate among scholars. Here is Shandilya Bhakti Sutra no. 83.
सैकान्तभावो गीतार्थप्रत्यभिग्य्नाज्ञानात् || 83||
Sa Ekanta Bhava Gitartha Prati abhijnanat
One who understood the essence knows that true purport of the Gita is
exclusive and unalloyed devotion to Godhead.
This sutra is pointing towards the Bhagavad Gita Shlokas 6. 30 , 9.22 ,9.34 , 11.55 ,12.6 etc.
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