Exactly, or open the protocol and let the community write it.

Third option is to ensure the downloader runs under proton, which I think it does but haven’t tried.

Protocol is well documented already, GOG aren't really blocking community clients:

https://gogapidocs.readthedocs.io/en/latest/

The problem is mostly that their backend isn't wired for Linux builds so you can't use the APIs for native Linux versions.