Easiest way to put money on my apple wallet

Gift Cards cannot be redeemed for cash, so you will not be able to put it on an Apple Cash card or any other card in the Wallet app. Gift Cards can only be redeemed to your Apple Account balance and used for purchase as detailed here:

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

Easiest way to add funds to Apple Cash is to give a family member or trusted friend $20 and ask them to Message it to you. The Apple Support articles I linked below explain in detail how to transfer funds using Apple Cash.

Send and receive money with Apple Cash - Apple Support

Add money to Apple Cash - Apple Support