"We were bummed that these iconic games weren’t available to our players, so we‘re very happy to work with the crew at GOG.com to rectify that,” said Rob Bridenbecker, Blizzard vice president and executive producer. “This has been a long time coming and we hope our players will be excited to jump back into these classic titles.”
Nice
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"Q: Why are there two versions of the game?
A: Diablo is available in its original, unchanged Battle.net-compatible version as well as a GOG-enhanced version.
Q: What is different between the Battle.net and GOG-enhanced versions of the game?
A: The original Battle.net version of the game is completely unchanged and allows players to connect to one another through Blizzard’s Battle.net online-gaming service.
The GOG-enhanced version offers several quality of life additions, including minor audio fixes, up-scaling support for resolution and refresh rate control, and compatibility fixes. Please note that this version cannot connect to Battle.net and only supports multiplayer via LAN and P2P connection."
That part worries me a bit. If it is like that for war2 I think anyone connecting to battle.net would have a screwed up screen.