The GOG version of Diablo features touched up visuals and support for HD resolutions, as well as numerous compatibility fixes that allow the game to run on modern systems. There are two versions of the game available. The first is almost identical to the original Diablo and is compatible with Battle.net, while the second is the GOG version. While this version has updated visuals and technical fixes, it cannot connect to Battle.net and only has multiplayer through LAN or P2P connections.
The Diablo III blog clarifies that Diablo won't be available for purchase on the Battle.net App and that there are no plans to release an updated version of the Hellfire expansion for the game at this time. There are also no plans to release an updated version of Diablo II or Diablo II: Lord of Destruction in the near future.
Diablo isn't the only classic Blizzard game that will be available to purchase on GOG in the future, however, as updated ports of Warcraft: Orcs and Humans and Warcraft II: Tides of Darkness will be released for the service at a later date.
Blizzard has been making an effort to update its back catalog so that its games are easier to play on modern systems. For example, Warcraft III: Reforged is also due to be released this year.
The original Diablo was a great game when it was first released, but it was eventually eclipsed in by its successor. It's a shame that Blizzard has yet to announce a remake or updated version of Diablo II, as that game still has a following and continues to receive fan-made mods to this day. If an updated version of Diablo II had been announced for mobile phones, then the fans would likely have given it a far more positive reception than the one that they gave to Diablo Immortal.
Diablo is available to purchase on GOG.com right now.