welcome to giffgaff and the community
• If a network pin is being asked for???
• If so the probability is , that your phone is locked to a network witch is not 02 or giffgaff
• And therefore will need unlocking before use it with your new supplier
• If the phone is locked then your best way is to contact the mobile network provider the phone is locked to and request to have it unlocked
• Also here is a good guide to help ⬇️
I hope this helps;