Topic on Talk:GRID Autosport
My friend had to walk me through it, I never used Steam Depot before so I'm afraid I can't help you with that - however, note that even by using this method you need to login with your Steam account and own whatever you're trying to download. Therefore you wouldn't be able to DL this DLC if you do not have a valid license.
With that said, you can download the files and drop them in your game folder, it will replace the "normal" resolution textures, it should work. I added an entry with a MEGA link on the game's page. It won't "activate" the DLC though, I'm afraid that it's gone for good if you don't have a license already, sadly. Let me know how it goes ;)