The authentication code is a 6-digit number which is sent to your GCash-registered number. This is used to link your GCash number to your device, allowing you to save your GCash credentials for 90 days. This means the next time you log in to the App, you only need to enter your GCash MPIN.
For your security, after 90 days, you will need to authenticate your device again. You will be automatically asked to enter your mobile number then input a new authentication code to proceed to log in.