This is a fix for Benny's (qb-customs), not JG Advanced Garages. You will need to take your vehicle to Benny's and re-apply the livery for your existing vehicles in order for this to take effect!
This is an issue that can be fixed by making a small modification to qb-core
Go to [qb]/qb-core/client/functions.lua
Search (by pressing CTRL+F) for the code below and remove it:
andGetVehicleLivery(vehicle) ~=0
The function you modified should now look like this:
local modLivery =GetVehicleMod(vehicle, 48)ifGetVehicleMod(vehicle, 48) ==-1then modLivery =GetVehicleLivery(vehicle)end