You can opt out of auto updates and betas within the Galaxy client, by right clicking the game and going to "Configure". In here, you can disable beta input or role back versions or back to new versions. I do not own Bannerlord there, but you can see if you can opt in right now by right clicking the game and checking what versions are available.
As for updates themselves, it varies by game, but most take a while after the updates come to Steam. This probably isn't the case for Bannerlord, so it is worth checking within the client.
Good Old Games. It is a digital game seller site, with a dedicated forum/review system and now a game client (called GOG Galaxy, or just Galaxy). They are owned by the same people who make the Witcher series or Cyberpunk 2077. They offer games
without DRM, meaning you truly own everything you buy and always will.
They are a great place to buy games, because whatever you buy, helps support developers
far more than Steam or Epic or Uplay etc. They also offer customers a 30 day return guarantee, which is always good.
Sadly no, GOG doesn't have betas.
Yes they do. It just depends on the game for what is actually there, but GOG does provide it. You access this information by right clicking and go into Configure. For example, what it looks like for Cyberpunk 2077:
Unless you meant GOG Bannerlord specifically?