Not wanting to HJ the thread but settings are the biggest problem seen on giffgaff. Have been with them since 2010 and hard to beat for price and you get the chance for payback too (real £'s) to help with the razor habit should you have one...
With regards to the 'community' that is the port of call for non-account issues such as settings etc but you will have found various answers to the same question.
You need to use a dongle that is O2 or unlocked, as giffgaff is owned by O2-Telefonica and uses their masts (MVNO). Try
HERE for dongle settings and see if that is a little clearer?
If you are using an exclusive SIM for your laptop you can't use a normal 'goodybag' but have to buy a gigabag or use PAYG rates which, after the first 20MB (20p) ramp up (20p per MB).
NTS - yes, O2's data coverage is the worst of the big players and one of my and others' gripes. It's ok offering unlimited data when many people can only get 2G!
Check data coverage here: http://www.o2.co.uk/coveragechecker# though with the digital switchover, O2 were meant to be rolling out more 3G - just in time (no doubt) for when 4G is the norm!
Ironically, I switched from virgin as service from them was very poor and missed important calls.