GiffGaff does not offer postpaid numbers, so this method will not work for F2P Overwatch 2 accounts.
https://community.giffgaff.com/d/22835585-requirements-sim-only-deals/2
Giffgaff do not offer postpaid or contracts.
All giffgaff services are pay in advance / prepaid and you can leave any time.
You can join from abroad, or when you get here.
There is no credit check or UK resident requirement as it is prepaid.