Good way to spend your time in lock-down. And also a good decision to stay home and reduce contact between ppl like that.
For your question: here you have a guid of painting a car in GTR2. GTL is basically the same, just the files need to be called *.GTL instead of *.gtr. Also you do not need the graph and the tech files at all, GTL does not use them.
The programs you need are a painting software (most usw photoshop with a dds plug in. Newer Version can open dds files anyways) and the geditor to open and save the *.gtl files. The link in the guide is dead but I guess you can download it somewhere, it is freeware. If notyou can ask again here, you'll get it another way then I guess.
Keep in mind thatthe car in the guide is an original car of GTR, not a Mod. Some mods use a bit different structures. Many have less *.gtl files (Ultra_tex and Cpit are often not used). The best thing is to look how the structure in a number folder of a car looks like. If there are *.gmt files in the folder you should stay away from them and pic another number folder, because you need to have more software to work on them (if you have the software you can do what you like, then just ignore what I just said).
Hope you can paint something nice, feel free to show a pic or two
Stay healthy
here is the guid:
https://www.racedepartment.com/threads/gtr2-
skinning-guide-how-to-get-your-car-in-game.18524/