As a rule 📏 of thumb giffgaff do NOT lock phone 📱 or sell locked phone 📱.
HOWEVER due to Apple 🍎 and their stupid rules ALL iPhones are sold unlocked BUT lock to the network of the first sim card instead upon purchase.
So as giffgaff do NOT lock phones then the iPhone locks to patent company and signal provider O2.
Currently you have to purchase an O2 UK sim card and top it up 👆 and use it for a month or two before O2 will unlock it for you.
However I suggest that as giffgaff now sell I iPhone 5C and 5S them giffgaff should now allow all giffgaff iPhone owners to contact O2 customer services and pay a minimum of say £15 to get it unlocked officially by O2