In short, No.
giffgaff is a UK MVMO that means they run off o2's network as you know, however it also means they don't own any masks ect in this country let alone abroad. So for every call/text or MB of data you use abroad giffgaff had to pay the network you are roaming on for. giffgaff has to pay a much higher rate for this, so therefore they also have to charge us a much higher rate for that service to cover the costs. giffgaff simply has no bargaining chips when it comes to getting a deal with these networks as there's nothing to offer in return.
For use in the USA I'd really recommend getting a local SIM there if you travel regularly, or use a network that has a good roaming agreement with those networks that makes it cheaper for yourself.
giffgaff is great in the UK and indeed the EU, but outside of that it's hit and miss depending on what the networks in each location charge.