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.
The sad and brutal truth is GMS itself could... but you can't do it yet, if it's your first project.
These projects are VERY complicated to start from scratch. If you're dead set on this idea, I would maybe check for an engine specifically tailored to make these kind of games to start with.
But even then, you really gotta learn to crawl before you learn to run.
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