Now for the easiest part. Install the content using by subscribing to every item in the Workshop Content Pack. You may need to log in to steam if you're opening in an external web browser. Once your download has completed, you can proceed to the next step.
If model detail is not set to High then cars in-game will look like spikes and spaghetti. This is not considered an issue with the content pack.
To start fixing your game, it is highly recommended that you uninstall all of your addons. You'll want to do this if you want to keep things simple and straight forward. There is a simple way to unsubscribe from all of your addons at once, as shown below.
Having followed these instructions, you are going to want to quit the game, if you haven't already. Then, purge your Garry's Mod folder of any remaining downloaded content.
To do this, you can right click your game in your library, click properties, local files and then Browse to be taken directly to your Garry's Mod directory.
Now click on the garrysmod folder and open it, and then you're ready for the next step.
With your Garry's Mod directory open, select the following folders by holding CTRL and clicking on them to select several at once:
If any of these exist in the Steam\steamapps\common\GarrysMod\garrysmod directory, hit delete on them.
Deleting these folders will remove any files that remained after you unsubscribed from your addons. If you don't have one or more of these folders, it's not a problem, just delete the rest. After doing this you'll want to launch Garry's Mod. The game will create new folders for the ones that were deleted, and you can now subscribe to the PerpHeads content.
If you've got to the end of this guide, and you still have issues with your content, then you should create a support post over on the forums or join the Discord Server and post a message in #help-and-support.
If you have any issues with content, ask in /help in-game or contact a staff member on the forums.