Treatment of sale of used car in gst annual return

Only taxable value of Rs. 3 Lakhs will be shown in GST returns.

Differences in turnover amounts between books and GST returns will be dealt in GSTR-9C

You need to refer to wdv of the asset rather than sale price and depreciation. 

Incase depreciated value is wdv i.e.3lakh then 2lakh happens to be taxable value on which would be considered gst inclusive