^^^Well it wasn't me, Hc4361! (I know relatively little about mobile phone networks).
GiffGaff is actually O2. (It's owned by them and uses their transmittters). It seems to be 'a Marmite thing' among users, with some loving it and others hating it:
https://uk.trustpilot.com/review/giffgaff.com
May 2015 Which? report on mobile networks...
"If you want to spend less on your phone and are comfortable with online-only customer service, Giffgaff is the provider to go for"