First off, do you understand the difference between auto topup and recurring goodybags?
If you currently have a goodybag, and want to replace it with another goodybag every month, it's recurring goodybags that you need, not auto topup. Auto topup will only give you credit for more expensive services that are not covered by goodybags. Worse still, it will not replace your goodybag when it expires.
If you let that happen and (in the worst case, if you have auto topup on) will start using credit at 5p/MB. That's probably not what you want at all, if you currently have a goodybag and don't already have recurring goodybags on.
ps -- you'll find both options at https://www.giffgaff.com/profile/payment-details