I ran GOG Diablo and after trying to connect to Battle net, it checks version and tells me an update is needed. After seemingly running the update in game, it prompts for a restart. A BNUpdater pops up with a message that this program is used to update Battle net and does not need to be run. After going back into the game to try again, it does the same thing so it seems to loop.

I would love to try and play online but this is frustrating.

Are you completely sure it's the GOG version that you're starting, and not the CD one that would happen to also be installed on your computer?

As far as I know, even though the installer itself has been updated several times since GOG's first release, there has been no update to be downloaded from Battle.net, and I only encountered that same problem when trying to connect to the new "Global" Battle.net gateway using my old CD version.

To purchase the gog version is preferred. You can connect with v1.09b but you will need to update the registry...

https://d1legit.com/2019/04/04/diablo-1-connecting-to-battle-net-gog-net-with-v1-09b/

If the GOG Diablo is telling you to update, you need to open the GOG client and verify game files.

Contrary to what Kougun.kW and the linked thread says, you CANNOT connect with the original CD version patched to 1.09b. You would need to also copy files from the GOG Diablo version to the CD installation first.

I don't have a CD copy of the game, only downloaded from GOG. The version installed from GOG says 1.09 V6. I have tried the Verify/Repair option. I tried to download the patch file from two different places and it tells me that an error occured, the patch cannot be updated. It is unable to find Program Files (x86)\GOG.

Как они лихо присвоили мою видео инструкцию себе :)

https://www.gog.com/forum/diablo/run_diablo_1_gog_no_classic_in_window_and_play_on_battlenet