Same here, so I can't even see the text it is supposedly displaying way off screen.

Если вы про классическую версию, вот видео
https://youtu.be/QQ71nq10vO8

I used this video: https://www.youtube.com/watch?v=ivHzD9SdCrg

What it wanted me to do:
1. Copy BATTLE.snp, bncache.dat, bnupdate.exe from the root of the game to the dx folder. This folder is used by new version od diablo created by GOG. Now you should be able to use Battle.net in new version of the game.
2. Change video settings of your game in a luncher from fullscreen to windowed, or windowed- integer scaling (this worked for me).
3. Add the necessary lines to the registry HKEY_CURRENT_USER \ Software \ Battle.net \ Configuration \ Battle.net :

gateways
connect-forever.classic.blizzard.com
8
Global

4. Use this instruction to add port forwarding to your router https://portforward.com/diablo/ . Its easy and even has step by step instructions for different router models at the bottom of the page. - this is not in the video

5. create a new txt flie on your destop. Copy this, but remember to change the first line to the directory of your game:

"Change THiS:\dx\Diablo.exe"
timeout 5
REG ADD "HKEY_CURRENT_USER\Software\Battle.net\Configuration" /v "Battle.net gateways" /t REG_MULTI_SZ /d "1000\005\0uswest.battle.net\08\0U.S. West\0useast.battle.net\06\0U.S. East\0asia.battle.net\0-9\0Asia\0europe.battle.net\0-1\0Europe\0connect-forever.classic.blizzard.com\08\0Globa l d1\0" /f

save the file. Change it's extension from .txt to .bat.

When your run the file. It will run Diablo with some regestry changes that will prevent from timing out. It worked for me. I hope they fix this, because it's only a workaround. It also shows that you can clearly play with battl.net with new version of the game. Also, the text will be not in the right place. It will be in a corner of a screen.

If you can't see the text and the game seem to be stretched (it should be 4:3) you may have to change some settings in your video drivers, to prevent text going off screen.

It's not an easy fix, but if you ignore cyrylica and just follow the video above you can easily make it work. Also remeber to set your port forwarding: https://portforward.com/diablo/. It's easy.

I'm not an expert and I was able to fix it.

Я и тему сделал в которой описывается способ играть по battle.net не в классическую версию
https://www.gog.com/forum/diablo/run_diablo_1_gog_no_classic_in_window_and_play_on_battlenet

В прошлом сообщении сказано как запустить классическую версию в окне.

:)

Google translate is a powerful thing.

These instructions allowed me to get the game to open in "windowed" mode, and the text on SOME things shows like in the video, however now the game is telling me that there is a required update when I try to get on battle.net and it fails to download/install.

2) Add the gateway in the registry (append below)
branch HKEY_CURRENT_USER\Software\Battle.net\Configuration\Battle.net gateways
value:
connect-forever.classic.blizzard.com
8
Global

Yeah, still doesn't work for me, unless I'm doing somethin incorrectly. I'm pretty sure I'm not... I can't get this game to run in windowed mode, nor show me ANY kind of text in the game.

Возможно
https://youtu.be/QQ71nq10vO8

Classic version cannot be windowed. It's not built for it. Anyone who says they have windowed it, and is playing on Battlenet, is straight up lying to you.

Dll files that allow Windowed mode for classic Diablo, mess with the version to such a degree that the file verification on battlenet itself rejects the game copy, preventing battlenet access until the dll is removed/replaced with a version the game came with.

it can be windowed with Dxwnd using the original ddraw, but there is really no point to since DX wrapper works on bnet.

if you want to go on battle.net with DX version, just copy BATTLE.snp, bncache.dat, BNUpdate.exe to DX folder. you also need to edit registry to add the Global gateway like sifaa mentioned.

Don't you need the menu option to be coded into the game for it to work?

Also I'm pretty sure it won't work, because the file verification for battlenet is very strict.

yes that is done with editing registry HKEY_CURRENT_USER\Software\Battle.net\Configuration\Battle.net gateways, by replacing the old gateways or adding to below:

connect-forever.classic.blizzard.com
8
Global d1

How does that fool the file verification of the server though?

there doesnt seem to be any verification anymore, Torch mod also works like before, even StrangeBytes ddraw was able to go on bnet GOG version.