You can get an instand PAC code from here (when logged into the correct account) https://giffgaff.com/support/pac-code-request
Once requested, each time you click on the link above, the PAC will show, which is useful, incase your forget it.
You have 30 days to use the PAC. If not used in 30 days, then your number will stay with giffgaff until another PAC is requested and used.
Once used, your giffgaff sim will no longer work (so back up contacts). Any credit or payback on your account is also lost once PAC is used, so you may wish to use up any outstanding balance/credit. If you do have any payback due, you may also wish to wait until this is paid out (June 2019) before transferring number.
Hope this helps? All the best with the new network.