If you want to obtain a PAC to leave giffgaff and take your number with you, either ring 43431 or just click http://giffgaff.com/help/pac-code-request
and you should receive your code via SMS within 2 working hours the PAC will also be shown
on your My Giffgaff page under your phone number
A pac code is only valid for 30 days, so if you don't transfer your number in that time, you will have to ask for another one.
Your giffgaff account will remain active, your sim will be deactivated and your account numberless, you will loose all credit, goodybags and payback associated with that account as soon as you port your number to your new network provider.
I wwould also Log in to your account, goto payment details and delete all payment card details.
Save changes before exiting