unauthorised apple service charges

If they are legitimate charges from Apple, you will see them here:

View your purchase history for the App Store and other Apple media services - Apple Support

If the charges are there then they were made by:

• someone that knows your Passcode and double clicked the side button with the amount showing on the screen
• a member in a family plan where you are the organizer and have allowed them to make purchases to your payment method

If the charges are not there, then your gcash account has been compromised. It has nothing to do with linking it to your Apple Account. Contact them to secure your account and dispute any charges.

If would also suspect that what you see is a phishing attempt and the messages are scams. Only you would know if the amount was actually deducted from your account and clicking any link in the messages that further asks for your personal/financial passwords is only going to give the scammers everything they need to take your money.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

I would contact Apple support. Usually, Apple will tell you what you're being charged for. On this, they don’t.