I love my steam controller, so I always add my GOG games to steam as a non-steam game launched using galaxyclient.exe and the gog gameid. This way I get the achievements and time tracking of Galaxy with my preferred control scheme.
However just recently I have found that games I add i that way to steam will not launch, the galaxyclient.exe will launch galaxy to the game page, the purple play button will become greyed out but there it stops. If I disable the steam overlay this makes no difference, but if I disable the Galaxy overlay the game will launch via steam, with or without the steam overlay enabled. But of course the galaxy overlay is the whole point!
Now this could be some sort of steam problem but given that the disabling of the galaxy overlay makes it work I suspect something in Galaxy, I have galaxy version: 2.0.80 beta.
Has anyone else found this? My most recent game to have this problem is Talos Principle 2, (bought ages ago but never got round to playing:) )

diziet

edit: my long installed GOG SkyrimSE which has been launched from steam using this method now also does not get beyond the purple play button being greyed out, unless I disable GOG overlay.
For some reason GOG Fallout4 (currently installed with FOLON) is working!