You need to enter your Government Gateway user ID and password to sign into your account for the first time only. If you do not have sign in details you can create them on the same sign-in page. Whenever you use the app again, depending on what your handset supports, you do not need to enter your sign in details again. It is much simpler to use the app as you can sign in using:
a 6-digit PIN
your fingerprint
facial recognition