A market maker is the counter party to all your trades. GFT, Oanda, fXCM, etc are all market makers/dealing desks.
A MM will try to offset your trade with a counter trade from his system. This, obviously, is all done using computers. If you are buying a 100k lot of euro, they will find a similar offer to sell on their system, first, to settle the trade. The 2-3 pip spread they pocket. that is how they make their money.
if they can't lay off the trade, it is they who have to go into the interbank market to buy or sell for their own account to offset a customer trade, at which point the customer is trading with the dealing desk. since every trade goes thru the desk, they act as counter parties to ALL your trades.
If still confusing, PM me and I'll go into more elaborate detail.
BTW, GFT is one of the better (top 3?) fx dealing desk brokers out there. generally, their clients don't get screwed. But understand their system - if you hold a trade overnight, they settle your trade at the close and re-open the trade at a new price to adjust for interest. it's a little confusing at first.