That's to be expected: they are surely different than default vehicles, so they cannot be recognized by the GSX installer. I'm not sure if this is possible, or even right to install: we would then have to "chase" each time they would make changes to that vehicles, and update the GSX installer each time.
The proper solution, instead, would be if REX would let you CHOOSE if you want to replace that specific vehicle, separately from the others (I'm assuming they let you choose if you want to replace the default ground vehicles)
