Sorry but giffgaff DON'T lock phones so can't unlock them either.
The only way to get it unlocked is to:
Buy yourself a O2 SIM card
Top up £15 minimum
Contact O2 customer service and ask for the unlock code and follow their institutions.
Only iPhone purchased direct from Apple or giffgaff remains unlocked for life.
Also it's best to get O2 to unlock as they will permanently unlock it.
Other unlock service could unlock it but on upgrading to next iOS update it could very well lock itself again.
In short it's locked to O2 not giffgaff