Can we stop charging my initial card as I changed the payment method already?

Are you talking about this app ?

• A mobile wallet app that lets you instantly pay bills, buy load, send money, shop, and more – all in the safety of your own home!

You can't use GCash app as your Payment method for the App Store.

See -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

As noted, you can't use Gcash with Apple. If you meant Apple Cash, you can use it in certain countries, but any charges already billed to your credit card will proceed to the card. You are welcome to contact Apple about this but I don't think they can undo that. If the card is now canceled then you may find that Apple will continue to try any valid payment methods it finds for your account and the charges will be made to the alternative payment method.