I also took the offer. But, I can't use the coupon because I'm already a Gaana+ user. I'll have to wait till November. The coupon is valid till December 31st, 2019.
Can't you use virtual debit card
. Paytm and Freecharge aren't asking you to add payment method, it's Gaana. Use any payment method and cancel it before 3 months. Btw, you can use Google Play balance and Paytm to subscribe. You can also manage and unsubscribe or unlink Gaana+ from these services.