You don't need to change anything in .gen file, except for search pathes that is
It's like with anything else - your new GMT files must be in search patch specified in .scn or .gen file. Shading texture must be copied aswell - it's generated by material tool for a reason
I find it most convenient to have one folder with all source GMT files and textures and export folders placed within car's or track's main folder.
For example:
...../MyCar/SrcFiles/
...../MyCar/MAS/
...../MyCar/MyProject.rfmtp
In .gen file add: SearchPath=......./MyCar/MAS
Or:
...../MyTrack/SrcFiles/
...../MyTrack/MAS/
...../MyTrack/MAS_Sky/
...../MyTrack/MAS_Grass/
...../MyTrack/MyProject.rfmtp
In .scn file add: SearchPath=......./MyTrack/MAS
In .scn file add: SearchPath=......./MyTrack/MAS_Sky
In .scn file add: SearchPath=......./MyTrack/MAS_Grass
And of course set all these folders in project options (also set /SrcFiles as texture path).
The only change you'll ever need is in .scn file when you export sky - you need to replace standard sky section in .scn file with the one given in manual.