how to refund my gcash

As far as I can tell, nothing was transferred. The payment failed because you had too little money.

https://help.gcash.com/hc/en-us/articles/900006222943-How-can-I-purchase-and-pay-at-Apple-Store-using-GCash-

Contact Apple. Click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> https://support.apple.com/HT201232