These are the specific t&cs http://giffgaff.com/boiler-plate/terms#Goodybags
5.14. 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);
c) in such a way that adversely impacts the service to other giffgaff customers;
If giffgaff reasonably suspects you are not acting in accordance with this clause, giffgaff reserves the right to impose standard browsing charges, impose network protection controls which may reduce your speed of transmission or remove the goodybag from your account or disconnect your tariff at any time, having attempted to contact you first.