This tool is amazing! Great job, it is exactly what's been missing in the GameMaker space!
I would start using it immediately, but there are a few things stopping me from adopting it:
I prefer setting the origin inside GameMaker using "Middle Center" for most of my sprites, because the size might change later and I don't want to calculate it for every sprite. My game has hundreds of little decorative sprites and managing the meta data inside Aseprite slices for them would be quite tedious. Inside GameMaker I can select multiple sprites at once and set Texture Atlas, Origin, Frame Rate etc. via the Inspector.
Unfortunately GM Link overwrites these settings with each update of the art, even if I choose not to use any of the Aseprite metadata.
It would be great if the tool could detect if the sprite already exists inside of GameMaker and update only the graphics and dimensions, keeping the rest of the settings in tact (unless the user chooses to overwrite it with Aseprite metadata).
The same goes for folders: Once the the sprites were created, I moved them deep into my folder hierarchy which is organized by biomes and further sub-groups like foliage, props, negative space, etc. Replicating that structure for every slice would be very tedious.
If a sprite already exists and has been moved to a folder, it would be awesome if it would stay put if no path has been specified otherwise in the Aseprite slice.
For small projects or if you use this from the beginning, it would be amazing!
If it was a little more flexible for different workflows like mine, I would use and buy/support this in a heartbeat! It's exactly what GameMaker needs because updating existing assets is a big headache!
Cheers!