You may need to wait a couple of hours (upto 24) for the sim to be fully activated before you can request a number transfer to giffgaff using a PAC from another network.
As long as you have confirmed the PAC is correct and valid with EE, then do not worry too much, try it again in a couple of hours or ideally in the morning.
If you still have problems tomorrow, then you should then contact a giffgaff agent who should be able to help you with this issue (if is is still there tomorrow). Use this link to message an agent (these have access to your account, unlike everybody who is on this forum) https://support2.giffgaff.com/app/ask
Hope this helps?