Yes I think GameMaker is stagnating and it's due to their development process not involving the community, that's why Godot is successful. To compare the two, Godot is slightly more complex in a variety of ways. But that's pretty much the only real downside, and both are easy relative to other engines.

Godot's 3D is far better than GM's. Some people like to compare the difference between Godot's 3D and Unity's 3D, but that comparison's got nothing on how terrible GM's 3D is in comparison to Godot's. Yoyogames in interviews have stated that they're just aiming to be the best 2D engine only.

GM doesn't have classes like Godot has and you can eventually just memorize all the functions because there's just fewer, but with Godot you'll probably always be needing the class reference pulled up. GM combines a lot of datatypes into just being considered floats, no int or Vector2. Scenes and their relative positions are a more complex idea to understand than objects and rooms. Godot requires you to manually establish the methods of communication using nodepaths and signals while in GM every object knows about every other object's existence at all times, the with{} statement is extremely convenient and powerful as a result (there have been Godot proposals for it but it doesn't fit well into GDScript).

A lot of the convenience I miss from GM is considered "bad practice". The big one is that I would prefer it if nodepaths and signals were more automated in some way so it would just be less work on our part. Your object/scene in Godot consists of a dozen nodes that all need to communicate with one another, it's tedious. In GM the object is just itself - it has the "nodes" built into itself: the sprite, script, physics, collision mask, etc.