I have made some assets, but I haven't made anything new in a while. I used to program a lot of 3D projects. Please check out "slayers upgrade" which upgrades the 3D functionality of Game maker. It's basically what I used to make 3D projects because there's a lot more math involved. I took all my code that I used a lot in 3D and packed it into that extension.
I also made a first person shooter engine which you can go through and see how useful the my upgrade is and see an example of how the scripts are implemented. The engines main features are a level editor and advanced 3D collision checking and response. The level editor can edit a heightmap and position game objects built into the engine. The heightmaps terrain can be edited with special tools. The color and alpha values can also be edited. There is a generator to create heightmaps with random hills.