Using GCash Mastercard is the just the same as using any other Mastercard Debit card!
For your typical transactions in retail stores or restaurants, ask to pay using a debit card then input your MPIN on the terminal when prompted by the cashier.
For online transactions, input the details or credentials being asked by the website.
You will get an SMS notification after you made the transaction.
NOTE: Official GCash representatives will never ask for your MPIN or One-Time PIN. Keep your account secure and transact only with trusted merchants and parties.