3G isn't avaliable in all areas, and some areas only have a 2G signal.
You can check whether you should have 3G service in your area here:
giffgaff was having trouble with internet signals a few days ago, that may still bea problem so check the service in your area on the community tab!
sometimes when I am at home I can't get a good 3G signal, some areas are STILL stuck with 2G signals even though 4G is coming out here in the next year or so (not sure when giffgaff gets it?) but if that's the case then i'm afraid there is nothing you can do until giffgaff/02 update their services in certain areas