GMS *can* do it, but chances are you won't make it anyhow.
Even *small* MMORPG's take hundreds of people working for years to make anything half decent. A single developer learning as he goes will probably never finish.
However, you'll need to be very familiar with GameMaker's networking functions, be familiar with networking in general, know a different non-gml language to write the server in, you'll need infrastructure for that server, and on top of that you'll actually need the game.