I've seen some mentioned, eventually after enough back and forth....
If you're looking for a single straightforward app that you can install it does not exist. If you are willing to get your hands dirty and are comfortable with building the ROM from source, you can bake it in yourself, but I highly suggest you either have someone help you or you know what you're doing. As previously stated above there are some things floating around that will harm your overall GoS installation as you are enabling many privileged permissions that goes against the core of GOS.
I do not have it on hand, and I'm not been able to fully test it myself, but I do have sources of people being able to enable Android Auto, while keeping those privileged permissions disabled, and the best part is you can accept an OTA update and it will not ruin anything.
That being said building a rom from source is not easy, it takes quite a bit of time and it can be frustrating trying to properly build and then bake in something you're even less familiar with.
It is possible, you can find it over on XDA if you know where to look or look hard enough. If anyone really really wishes to take a crack at this, let me know and I will post what resources I have on this but understand it is not for the faint of heart but it is possible