First do not use the GTL install under Steam. Keep that install as a 'clean install.
Copy the Steam GTL folder to a temp position.
Delete the content of the Steam gtl folder and sub folders.
On the steam menu right click GT legends and select priorities and Verify Local files.
It will then download GTL again and you will now have a clean unmodified install.
If somethings goes wrong you got the copy you made earlier.
Be aware that you sometimes have to repeat the Verify files.
GT Legends are not a Steam game as such and you do not need Steam to run to use it.
Make a folder on a drive something like D;\games\GTL
Then copy the entire Steam GT legends into that.
In that new folder run GTLConfig.exe as Administrator and setup your video settings.
Create a shortcut for GTL.exe
Change the priorities for the GTL.exe in the shortcut to run as Administrator.
Now install the Mods into that new folder.
Check if there is a Read me included in the Mod about how to install.
But most Mods are installed by unzipping the archive If the root folder of the archive are Gamedata then just copying the content into the GTL\ Gamedata folder.
As an example:
Go to Gamedata\Teams\GTC-65|Lotus Elan\601, click on the "car" file and open with notepad. Scroll down to the line that reads - classes="SIM_FULL, SIM_GTC, SIM_GTC65, SIM_Elan, SIM_GTCTC65, SIM_MASH" This is where you define where the car will show up in your game. In this case the Elan shows up anytime you run the GTC-65 class cars. If you type in "SIM_GTC76" in this line, the Elan will be available for play anytime you run the GTC76 cars.
Each car must have one of the classes (SIM_GTC, SIM_GT65, SIM_GTC76, SIM_TC65) in the class line to appear in game.
You can also create new custom classes (I have six different classes with specific cars in each)which we can discuss at another time,