Don't use a gog galaxy custom executable if you want to enable DX12 in aor. Create a shortcut for aor with "-force-d3d12" and launch aor separately from gog galaxy.

Custom executables in GOG Galaxy are apparently super broken and launching the game through a custom executable enables cloud sync (even when it's disabled) and disables GOG overlay, so you can't take screenshots. Probably a bunch of other stuff that runs in the background gets broken as well. I have ticket with GOG from April with no updates from them.

The best way to go about it is to:
1. Launch gog galaxy
2. Launch aor from the custom shortcut
3. After aor loads completely and you're in the main menu alt+tab to gog galaxy and press PLAY on the aor game page, so that gog galaxy actually knows aor is running.

You should just ask the devs to make an option for launching the game with DX12 from Galaxy.

Until then here is exactly what you want. DX12 with working overlay and optional cloud saves. Just download the file below and follow the instructions:

https://workupload.com/file/mfn3rvqWMP3

How to install:

1. Exit Galaxy. (Close it completely)
2. Extract the downloaded file into the game's installation folder and overwrite the existing one.
3. Now move the whole installation folder to a directory on the same partition where Galaxy isn't looking for your installed games (In my case i moved it to the desktop)
4. Restart Galaxy.
5. Move the installation folder back to where it was before.
6. Search for the game inside Galaxy. (It'll show as not installed)
7. Right click on the game's cover, "Manage installation" and "Import folder". Select the game's folder and add it to Galaxy again.
8. Start the game from the "Additional executables" menu and enjoy it with DX12 and working overlay. :)

Keep in mind that you have to do this again every time when the game gets updated.

Interesting approach. Would you mind explaining why this results in a different outcome than using the Galaxy GUI to add the custom executable?

With this method Galaxy "is thinking" it is an official additional executable and therefore the overlay gets enabled. The reason why you can't have overlay with custom executables added by users is that some games don't work properly when the overlay is enabled. But the cloud sync issue when using custom executables is definitely a bug which should be fixed.

I just noticed that art of rally still uses the old .info file with the buildId in it. That means when the game gets updated you have to copy the buildId from the updated .info file into the modified file. Otherwise Galaxy will show you that an update is available.

Good to know! This is super useful! Thank you!

You're welcome. :)

This workaround is no longer necessary. The option to launch art of rally with DX12 has been officially implemented with update 1.3.0. Thank you very much @funselektor! :)