None of their current plans appear to offer tethering. Are you on a grandfather-ed plan?
Not sure what the deal is with iPhone (I'm on their £12 option). All the other plans expressly say "Tethering is not permitted ..."
So they might need to pay Apple some £££ to get the ability to simply unlock tethering for GiffGaff's users.
I suspect that an jail-broken iPhone could tether on GiffGaff just fine.
Apple obviously provide the infrastructure for enabling/disabling tethering but I cannot see how it is in their interest to discourage it, except to keep the carriers happy.