I spent some time experimenting with my Blender addon and GameMaker's 3D the other night, and I realized that the models it was creating were not entirely how they should be. It seemed that if you used the option to flip the object's Y axis on export, which is often necessary because GameMaker and Blender's world axes are different, then the model's normals would be flipped.
This wasn't immediately obvious to me, since the correct orientation of GameMaker's built-in models has always been kind of ambiguous, in my opinion. After following the code in the manual to make some of the primitive shapes however, I compared the results to what my Blender addon was creating, and came to the conclusion that the output from my addon was not quite correct.
So after a lot of tinkering (Python and Blender API are not my strong suits) I believe I've finally got the addon updated to export models that are oriented correctly and which should always have correct normals. Additionally, I added a feature which a friend of mine had built into his own version of the addon, which will let you output the model script in an alternative way, to more easily allow you to add it to an existing model in your game.
You can download the updated version of my Blender addon at the original page, which I've changed to include the newly updated addon. That will always be the place to go for the most recent version.