The issue is the old packs did not all have those cars working up to date with Gmod, so it was good DMJ reuploaded that stuff. While we can say DMJ did not credit others, at the same I do not remember the other car uploaders originally crediting the people that made those car models for their original games like NFS and GTA San Andreas before they were ported to Gmod. However, to avoid getting this taken down, yes it is a burden Jacob, but crediting the first uploaders of these cars is the only way to play it safe, because this is a useful set of drivable cars.
Worst case scenario, Jacob, in addition to already having released this giant set of actual drivability for these cars, you just release those scripts you made making these cars drivable, as another DL, and since it would require players to already have the cars to begin with by whoever made the models first, that should not be a problem. I just hate to see this set of working wheels get taken down.