Hi Cosmin,
I created a six-sided cube with 24 triangles.
I have pasted a pastebin link below, of what the example export looks like. I believe the very first line means the version number which can be left as is.
In GameMaker 1.4.9999, I can load the D3D model using this script. For example: cube.d3d
A cube In D3D GameMaker format
https://pastebin.pl/view/4740a2a1
Script
https://pastebin.pl/view/7e678b29
Let me know if you need anything else. Thank you for doing this and take your time :)