Do NOT try 5555. @weeman - that's dangerous advice!
If you use the wrong PIN more than a few times (with some phones that's as few as three times), your phone will become permanently locked.
The PIN is the one needed to unlock the phone such that it can use the network of the SIM you have inserted. It's not something that's known to giffgaff - it's only known to the network to which your phone is locked.
Try contacting the network to which the phone is locked. If you bought it from a network, it'll be that network to which the phone is locked. Otherwise you'll need to work out to which network the phone is locked. Where did you buy the phone?
Dangerous????Really?I think you're being a bit over dramatic there mate.
I've unlocked loads of phones over the years with unlock codes,and I've never seen 1 that uses 4 digits to unlock a phone,that's why i suggested the gg sim unlock code,thinking the OP'er has got mixed up.
Besides is it not a case of just contacting the network and getting a PUK code if the phone getslocked out?