What provides the hotspot service? Is it a phone with a giffgaff sim, or some other device with a giffgaff sim?
If it's a phone, can you verify that the phone that provides the hotspot service can itself use data and is obtaining that data from the giffgaff network, not from a local wifi service.
If it's some other device, try temporarily transferring the giffgaff sim to a phone (making sure that the phone is unlocked or locked to o2) and verify that it can retrieve data from giffgaff.
If it looks as if the sim is no longer receiving data from giffgaff, check for possible temporary faults by putting in the full postcode (not a town name) at https://www.giffgaff.com/coverage-and-service#network and reading the statement immediately under the Live Results heading.