One thing I also notice...
Is that in GM it seems you are regulated to squares (tiles) when you design the game.
Does GM run into freedom limitations when moving an object.
It seems in Fus2.5, you can design in "free-form" (on a blank canvas)...... which I really like.
GM defaults to using a grid and placing objects in a grid but there's a shortcut to placing an object not aligned to the grid. They have quite a lot of features hidden away to simplify it, but it's still a slow process compared to Fusion.
Fusion has options - you can have a grid, snap to the grid or turn it off.
Another thing I forgot to mention that is a huge positive with Fusion is that you can run it frame-by-frame, so if you want to test something quickly you just click a button, run the frame and test it. GM has nothing like that - you have to compile the game every time and it will save the game before compiling, whether you want it to or not, and that process - save - compile - run - takes several minutes compared to Fusion which on my machine is instant.
Fusion & GM use INI files similarly, there's no real difference.
With both you buy the core program and export modules - html is an export module with both of them.
With both you can create your final app without further processing - so these extensions and C++ stuff people are talking about are features you can use, but they're not stuff you need to know about in order to create or compile an app.