Hi, I'm posting this because I have a possible solution, since I had the same issue:
Spine Version 3.3.07 Essential
Latest Game Maker version.
Furthermore, I suggest to install the English version of Spine, just in case.
· Export -> Json
Nonessential data (checked).
Pretty Print (unchecked).
Create Atlas (checked).
· Atlas settings:
All Regions options checked (5 options).
Padding X -> 2
Padding Y -> 2
Edge Padding (checked).
Duplicate padding (unchecked).
Pages:
1024
1024
1024
1024
Power of two (unchecked).
Square (checked).
Runtime:
Linear
Linear
ClampToEdge
ClampToEdge
RGBA8888
Output:
PNG, Atlas
Premultiply Alpha (Unchecked).
Bleed (checked).
Scale 1.0
Options:
Atlas extension: .atlas
All unchecked.
Ok, now a very important thing. Create a Sprite in your game, name as you want. And now load only the .JSON archive. Despite Spine generates 3 archives in the same folder, you only have to upload the .JSON. It will take a moment, make sure that there is no error message in the source code. Once the .JSON is uploaded, you will notice that you can't edit the image, and now there is a Spine button to open the Spine program. This means that Game Maker recognised the .JSON.
Never use the Shear option while animating, since is not supported in Game Maker and will give an error if you try to upload the .JSON.
In my case it only works if I put the 1024 number in all the Pages option, maybe someone else made it with a different numbers, please tell us if you know any other working numbers ^^