Why I am unable to add GCash as my Apple ID payment method ?

You can try this to use GCash as your standard payment method

1. In your phone Settings, tap your Apple ID/Profile.
2. Tap Payment and Shipping > Add Payment Method.
3. Select GCash as your payment method.
4. Tap Authorize.
5. Enter your GCash account information.
6. Enter the 6-digit authentication code sent to your GCash registered mobile number.

if this is not working it could be that gcash is not supported in your country to use as payment method