Auto topup is a dangerous feature if you are using PAYG rates. If you do want it activate here: https://giffgaff.com/profile/payment-details.
If instead you wish to have a goodybag start automatically, set up Recur goodybag check this guide: https://community.giffgaff.com/t5/Using-giffgaff/Recurring-goodybags-guide/ta-p/4753858
Below£3 how to automatically top up
mentioned, are you meaning auto goodybags (recurring goodybags) every month from a payment method? If so follow the link already given
'auto top up' is a completely different feature from recurring goodybags and tops up your airtime balance when it drops to £3 or less
explanation of that feature is here