I'll correct you then
It's a standard online credit card transaction, handled via moneybookers. You don't need to leave the GG site, you don't need to register with moneybookers, they just handle the work for GG. All you need to provide are the usual card numbers, card holder's name etc. You pay precisely what the list price is when you complete the transaction and that's it, unless your bank has a foreign currency transaction fee.
Just make sure you select the correct option, because there's a second moneybookers e-wallet type option where you do have to sign up and pre purchase credit with them. You want the credit card through moneybookers option instead.
As for steam-GG compatibility... I honestly have no idea.