gmt is a mesh (3D) file usually contained in a .gtr file, but often is loose in the folder. The .cas file references these when "building" the car; at the top you should see something like:
SearchPath=<TEAMDIR>..\
SearchPath=<TEAMDIR>
SearchPath=<VEHDIR>
//----------------------------------------
MASFile=FERRARIF430_OBJ.GTR
MASFile=FERRARIF430_TEX.GTR
MASFile=FERRARIF430_WHEELS.GTR
MASFile=FERRARIF430_cpit.GTR
MASFile=TALENT_07ALMS.GTR
MASFile=stex.gtr
The SearchPath (usually the same for any mod) tells the game where to find the necessary files; the "masfile=" denotes which .gtr files contain the .gmt and .dds files for that mod (thus you can have different cars and different .gtr files in one mod, with each car having its own .cas file since the latter is called by the .car file). As long as the .gmt or .dds files are in the path they do not need to be in a .gtr file; and the game will read the loose files before those in the .gtr file, which makes editing much easier. If your loose .gmt file is in the folder with the .gtr files, and is properly named (so easy to screw up and/or overlook) it should be read by the game.
Your crash is caused by INT_DRIVER_STYLE2.GMT not being found, or corrupt. If this file is not in the folder with the .gtr files it should be in one of those files, probably OBJ or CPIT. You need GEditor to view those (it also allows to import, export, and delete files from the .gtr).
FWIW, the "slot000" error is a game glitch from a temp file (vehgen.scn) created when you load a track, exit, and return; restarting the game clears it.