how can i buy storage

If you're referring to iCloud storage, you would upgrade - Upgrade your iCloud storage plan - Apple Support. As for the acceptable methods of payment you can use with your Apple ID - Payment methods that you can use with your Apple ID - Apple Support, "gcash" doesn't seem to be listed.

You are able to pay monthly for more iCloud storage, if that's what your looking for where you can store photos, videos and files etc. This can be done by going through your phones settings on your iPhone go to setting select iCloud (should be a profile picture, your name whilst saying Apple ID, iCloud, media and purchases underneath it.) Select iCloud again this can identified by the blue cloud, after that select manage storage this where you can choose your plan.