@RedLion3468 Good to create .GXT2 files I don't know, but in my Add-On cars I use original .GXT2 files from the game itself and just erase the names that are written in them and add the names of my cars, here's a folder with the dlc. rpf already with the .GXT2 file included, you just need to edit it and put the names of your cars. If you are still in doubt, I advise you to see this tutorial on how to make a complete add-on for gta 5
dlc.rpf file with .GXT2 file included: https://mega.nz/file/OtIRUCzK#j_JOsjbEFSfs90Yaml0Cem7bbdihgG7qeTML5GLXtow
Tutorial: https://www.imnotmental.com/guide/add-on-vehicles-tutorial/