The reason for this would be that you are either making a call to number that is not included in your goodybag allowed numbers, or if you abroad your roaming status is not on.
The goodybag allowances can be used to call any UK landline numbers that start with 01, 02, 03, and UK mobiles starting with 071 to 075 and 077 to 079.
A list of Inclusive and non Inclusive call, texts and services can be found here https://community.giffgaff.com/t5/Learning-and-giffgaffer-articles/Inclusive-and-non-inclusive-calls...
In order to roam with your goodybag you will need to have roaming enabled under your details page, check it here - https://giffgaff.com/profile/details#rlah
Make sure all toggles relating to roaming are set to ON - https://giffgaff.com/call-services