Sometime last year (I believe), I bought Stardew Valley from GoG (Windows). Checking on it now, I found out I have the 1.11 version, while game was updated to 1.3 or something like that. I have found nowhere a link to allow me to update my game to the latest version (I'm not interested in multiplayer, but in a >1.2 version to be able to use some mods, SMAPI slapped me on the wrist and told me my game is no good enough). Soo .. help?
The current computer version is 1.2.33, updated last Summer. If you go to your GOG account and look on your purchased games tab, you will find a download link for the current version. (I, too, have a GOG purchase of Stardew and I have just checked to confirm it is posted.) The multiplayer version has not been released yet and is probably going to be 1.3 once it comes out. Have fun!