If you are leaving giffgaff and wish to take your number to another service provider, you will need to request a PAC (Porting Authorisation Code) from giffgaff in order to give it to your new provider.
Once issued, the PAC is valid for 30 days (after which it expires).
Once you transfer your number out you will permanently lose any remaining giffgaff airtime credit, goodybag and payback points you've accumulated.
There are two ways to get your PAC code,
1) Online here https://giffgaff.com/profile/payment-details