There is no GST on Purchase of Such voucher to Recepient , yes GST need to be pay by Supplier who is selling such voucher.
Time of supply of voucher is cover by th provision prescribed in Section 12 & 13 of CGST Act . The voucher are taxable when : On date of Issue voucher : if the supply of underlying goods or services is identifiable at that point else on date of redemption of voucher.