Use TunnelBear. I can confirm it 100% works as i did it myself.
http://www.neogaf.com/forum/showpost.php?p=45851794&postcount=2319
No need for proxys anymore. You can turn the program off and on. Just open it when you want to buy from GMG, so it doesn't even have to run in the taskbar 24/7.
Heres a little preview on the interface. Just refresh a webpage once you have TunnelBear running and it will tick over to US prices.
I don't mean to sound so crass but it doesn't really benefit anyone to buy you a game on Greenman Gaming since GMG doesn't have a gifting system, so if they want a game in the future they can't rebuy it for themselves.