Another good reason is that The Witcher 3 apparently runs really well on it. Sauce: https://youtu.be/IWJUphbYnpg?t=9m33s

PS: Oh and I'm back on topic here. Just in case you found this horribly confusing

Open source is AMAZING! Games from my gog.com library WORK on Steam proton. I needed to edit my run scripts from my /tmp directory to point to the gog installer. Once the installation of the game is completed, I used winetricks against my proton installation to add dependencies needed by the game. I imagine you could achieve the same thing by running the installers in the game's redist directory. I plan on making a video tutorial outlining the process and posting it on Cup of Linux. I will note that the gog.com installer for DAO dropped an error at the end of the installation but the game appears to work just fine even with all of the mods I added to it.

Or you could try these: https://www.gog.com/forum/general/adamhms_linux_wine_wrappers_news_faq_discussion :)

You can use Steam proton EXTERNALLY to install any game including your gog.com installers. I made a video today showing how you can use Steam Proton using PlayOnLinux. I am not permitted to post a link to the video, but you can see it on YouTUBE - Spatry's Cup of Linux it is easy enough to find.