GoG gateway is not working and they most likely won't fix it anytime soon (if ever). I thought about offering an explanation on this, but it would be a long read that would probably just be ignored by most.

Anyways, the original gateways: US West, US East, Europe, Asia still work just fine. I personally prefer playing on US West as my connection is much better there. You'll need to use the original retail copy (from the cd-rom) because the GoG version gets rejected from the original gateways. If you don't have the CD, you can pirate its ISO and not feel bad about it seeing as you already technically paid for the game.

As a side note, if you want to have a single copy of Diablo that can access both Classic and GoG gateways, you'll have to use the original cd-rom copy and MANUALLY patch it to 1.09b. Do not do it automatically by logging into battle.net.

Then whenever you want to play Global, you can open up regedit and add in its gateway info. Logging into Global gateway removes the classic gateways from the registry. To restore them, you just have to clear the gateway registry and login again.

More info about it can be found here:
https://d1legit.com/2019/04/04/diablo-1-connecting-to-battle-net-gog-net-with-v1-09b/

You have an explanation as to why the GOG Gateway is down? Source?