GMS is an excellent 2d game engine. 2D being the key here
While 90s retro 3d quality can be achieved with it (Daemon Detective Racing Zero has that nice "DOS 1994 Magic Carpet" quality feel to it), if your needs are anything which even approach current 3d game visuals, you will be fighting a lost battle. Nor does it support (as I learned the other week) OpenGL shader 3 or even 2 - GLES 1.1 is supported only. This by itself limits what you would be able to do from a 3d visual quality point of view.
3D is not its strength nor do the visual tools support these very well, and you will be battling the engine's limits, and its severely limited 3d tools and commands. Let alone importing a rigged 3d character and tying it to a state engine which triggers the animations which then smoothly and seamlessly blend from one character animation to another.
Of course, you could re-invent the wheel, and build your own 3d character engine in GML, right?
Not. A. Good. Idea. This is just wasting your time, and I would strongly suggest looking at a modern well supported 3d game engine which have these things already figured out for you.
In short: unless you intend to create a retro type 3d game, just don't bother with 3D in GMS, in my opinion. Look elsewhere.
You mention you wish to create a RPG/strategy game in a similar vein as X-Com. The original X-Com is 2d isometric, and this is very doable in GMS. It is relatively simple to render your characters as 2d isometric animated sprites in Max, and build an isometric game. In this case no need for true 3d characters. It will vastly simplify the scope of your graphics as well, yet still look very nice (if executed well).
Secondly! Many strategy games rely on an intricate well built and user friendly GUI.
To remind yourself how diverse and deep the original X-Com's GUI is, play it online:
https://epicport.com/en/xcom
"But." And this is relatively large "but" depending on how much your time is worth to you:
GMS does not have any built-in support for standard GUI components. As I wrote, strategy games tends to lean a lot on GUI components. When building such a game in GMS, you will have to code a custom GUI system from scratch, and this does put a strain on your development time.
Coding complex expansive GUIs for a strategy game can be a daunting task all by itself. I wouldn't want to think about tackling an X-Com interface with all its screens without at least some help from the game engine itself.
Other 2d/3d engines come with a standard 2D GUI system out-of-the-box, and this can save a boat-load of development time while developing GUI-heavy games, because it avoids re-inventing the wheel once more.
That said, it can be fun to create your own 2D GUI components and system. And it gives you full control. Up to you.
Third, path finding. GMS's path finding only deals with stationary non-moving objects, I believe. Although, with an X-Com derived game you will have to develop your own game AI anyway.
In any case, building a X-Com like strategy game for your first game is (most probably) setting yourself up for failure. Instead of worrying about whether you will be able to import your 3d characters properly (which can be a nightmare to set up for a beginner even in a dedicated 3d game engine with direct support), perhaps try to get a basic 2d version using basic shapes running to see if you are able to create the basic strategy game play.
The original X-Com developer (Julian Gollop) created a bunch of smaller 8-bit strategy games before tackling X-Com with a team of people. Start small, see if things work out, then branch out.