There are a few things that you can try of which fixed this exact problem for me a while ago:
1. Delete the TDMCars SVN from your addons folder. Although I believe you have already done this.
2. Type mat_specular 0 into console as
@Sgt.James commented.
3. Delete all addons from within your folder and un-subscribe to them all. However rather than subscribing to the perpheads workshop collection just join the server
(making sure you allow custom files to download) and then it should download all the content you need for you. Just as long as you don't have any content in your folder then it should be absolutely fine. As a plus it will also make your Gmod boot and load faster as it is not loading any addons
4. Just follow Xquality's thread completely as that seemed to fix a lot of problems for me.
(Credit to @Xquality for his thread here.)