Is there any way to know what patch GOG games are on before buying them? Banners of Ruin is on 1.1.9, which has new content. I'd like to know if the GOG version is at this patch. Thank you.

Best way I know of is to check the GOG Database (gogdb.org); the "Builds" tab will list the current version. I can't post links, but I can tell you that the current GOG version is 1.1.16.

EDIT: As of 2022-01-28 the GOG version is 1.1.30.

GOG version is now 1.2.39 with the release of the Powdermaster DLC. Patch 1.2.42 shoul be coming soon.