Once you have linked your bank account, simply open the GCash app and click Cash-In > Online Banking. After that, just choose the account you have linked and follow the instructions from there. You will be asked to confirm the transaction and you will receive a confirmation message once the transfer is done.
For some banks, you will be sent a one-time pin (OTP) to confirm the transfer. Remember that you must not share your OTP with anyone. GCash and BPI will never call or text to ask you for your OTP.