Is your mobile data enabled? As this can be associated with background data. Apps running in the background can occasionally make use of the mobile data or it could be apps being updated in the background.
It is recommended to switch off the data option under the device settings when the mobile data is not in use or update apps over a WiFi connection only.
if you feel this is incorrect then I suggest you ask a giffgaff agent to investigate. To contact an agent please follow this https://giffgaff.com/support/questions