Hi there, you can't topup a goodbag. You can buy a goodybag or you can buy topup or you can do both. Goodybags only include "free" calls to most 01, 02, 03 and 07 numbers (excluding ones that aren't to actual phones, for example ones that go to pagers and messaging services that are not a phone).
If you are trying to call a 08 or 09 number that is not included in your bag (for example 0870, 0844 are not free) then you need to add topup credit (money) as you will be charged per minute. Also, please remember you can only have one bag at a time. If you had a bag and you used it all up then bought another one, the second bag will start on the date that was listed three times during checkout and it will now be queued to start when your current bag ends. You can start it early by clicking the link in My Giffgaff under your current bag if you want to end your current bag early and start the next one early.