The GV module in Ginesys is creates GV numbers serially. Anyone can duplicate the physical card with a serial no. and can redeem it. Chances are 1 in 100, but very much possible in case of GV sold through HO as those are all sold/issued at one go.
Requirement is along with Serial GV Number there should be a random no. Both has to be validated while redeeming. Solves the issue and OTP isn't a option.
The purpose of GV sale is to gift those vouchers to customer's friends & family. While redeeming, any random password / key generated are being verified that will be attached with buyers mobile no. Hence, the person who's a valid redeemer will not be able to use it for further purchase.
Marking this as will not implement. We appreciate your effort. Keep posting.