Sant gyaneshwar (dnyaneshwar) was a famous saint from maharastrian bhakti movement (varkari).
Whose avatar was he ?
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According to Navanath Bhaktisaar (नवनाथ भक्तिसार) Chapter one -Shloka 48- Shree Gynashwar was avatara of "Shree Vishnu " in the form of Shree Krishna. i.e. Shree Krishna himself incarnated as Saint Gyaneshwar in Kali-Yuga for the welfare of people.
It's said in NavaNath Bhakti Saar that at the end of Dwapar Yuga and starting of Kali Yuga. Shree Vishnu in the form of Shree Krishna called upon Nine Narayan Rishi's from Swarga Loka and instructed them to take incarnations as NavaNatha for the welfare of people as Kali Yuga is approaching and due it's bad effect on people . Shree Krishna also told them that ,he himself will incarnate along with them and will be known as Gyaneshwar. i.e. GyanDev which is his other name.
Shree Krishna told Nav-Narayanas thus-:
मी ही प्रत्यक्ष जन्मोन । ज्ञानदेव नामे मिरविन ।।48।।
Mi he -I myself ,Pratyaksha - In my real form (i.e. Vishnu) , Gyanadeve Naame - As the name Gyandev , Miravin - Will proudly roam.
I myself will incarnate as Gyanadeva and I will become famous by this name.
Here we can read NavNath Bhakti saar in (Marathi).