Take a sheet of paper and a pen while you are browsing cars in menu, and make columns with GTLM and GTE (write car number/manufacturer/team/year) then it will be easier for you to set classes.
I understand that your gdb menu is
GT3 = GTLM/GTE
But you want to split in 2 so you should write in menu :
GT3_GTLM = GTLM
GT3_GTE = GTE
Then you should do the changes in every CAR files like :
Classes="GT3_GTLM, ..." OR Classes="GT3_GTE, ..."
Moreover you can add the year like 19GT3_GTLM or GT3_GTLM19 by example