All of the collision in this engine is based on simple shapes (which are generated in-engine): axis-parallel cylinders, axis-parallel cubes, and axis-parallel blocks whose top and bottom are bilinear interpolated planes ("slopes" for short). There's no support to load 3D models externally, all geometry in the GMS1 version is generated in-engine using path assets to describe shapes. (Very inefficient, but this was before I learned Blender and I desperately wanted to avoid 3D modelling if I could)
The GMS2 version also adds a global terrain which is generated using an image heightmap (the idea is to cut down on the number of base shapes needed to represent the level geometry, since generating them is so slow) and a more exotic "path" terrain type which is basically a sequence of connected quads with arbitrary positioning - perfect for roads and paths (if they're not very tall and closely aligned to the ground) and curved walls (when they are tall).
Collision checking is the biggest performance hog so ideally you should use cubes and the like as hitboxes whenever possible - even with the stuff done to make this more efficient (using a precomputed hashmap for static terrain) the sheer number of points that needs to be checked makes it take up a lot of processing power.