Giffgaff do make it clear that...

...Goodybag data use is for use on your mobile phone and that it cannot be connected to any other device:

"5.13. In addition to our standard terms and conditions, all usage must be for your private, personal and non-commercial purposes. You may not use your SIM Card:

a) In, or connected to, any other device including modems, dongles or any other way to connect to a PC (unless you are on a gigabag plan)

b) fraudulently;

c) in such a way that adversely impacts the service to other giffgaff customers; or illegally"

They don't specidically use the term "tethering", so it isn't limited to *just* connections to a PC.