I've enabled the beta channel, but still can't see version 1.3 as being available. I've also tried uninstalling and reinstalling the game to no avail.

Is there anything specific that has to be enabled or done to get the multiplayer beta on GOG Galaxy?

If I go through the website I only see the linux multiplayer version and I'm looking for the Windows one. Thanks for your help!

heres the news post for it
https://www.gog.com/forum/general/stardew_valley_now_has_a_multiplayer_beta_you_can_try_b4cf5