Game Maker Server doesn't support GMS2 currently, the developer has been working on a re-write of it for about 2 years now.
You're best off making your own networking functions, or using a method that won't go out of date. As by the time he gets it working with GMS2, something else will probably break it.
I used it for GMS1.4, and it worked quite well, but it still isn't as good as becoming well acquainted with networking and making your own system specifically for your game. And you'll have control over everything then.