I don't know if this is still in active development, but it would be great to see support for slices... if you have an aseprite file with a big atlas of environmental assets, for instance, being able to update them in GameMaker with a single click would save so much time.

I suggest creating a feature request here.

If you still need this, I've hacked together a solution using an extension for Aseprite. It's a modified script from Gaspi that, in my version, exports all your slices into separate .aseprite files that will then be caught and updated by AseSync. You just name your slices "spr_mysprite" and if you need subdirectories you can add that in "user data" on the slice as "folder1\folder2\folder3". It works with animations too. Anyway, just putting this out there in case you're interested.