You can buy gv from that cc bank portal like hdfc smart buy , axis gyftr , amex gyftr etc etc or by other portal like phone PE , gpay recently then add in GV ..
These gv can only be used in Amazon & amazon associated pay link ..like Domino's, jio, swiggy etc etc .& For all bill payment in Amazon app
But not on repayment of loan product
You can't get bank the money in bank
If you load amazon wallet using UPI or dc from bank account...
You can withdraw and get back to bank /bank to bank transfer like u do in UPI
I hope this got all your doubts clear