ChrisGriffin: Warcraft 2 got it.
Rather not. Both Diablo 1 and Warcraft 1-2 landed on GOG because Blizzard is not planning "merging" them with remastered versions, like it was already done with Starcraft and will be done with Diablo 2 + add-on (basically because they won't make any remastered version of those games).
In other words - those games are not planned to be "merged" with strongly integrated with Blizzard's client remasters, but are still Blizzard's property, too "troublesome" to be sold in their own store (partially due to their policy of making remasters, but probably also because they're not willing to release patches anymore for those games), but here is the GOG's role - as external "hub" to sell those games.
It's just my guess, but I'm guessing that Blizzard is not very "DRM-free friendly" company, but those games (Warcraft 1-2 and Diablo 1) they've considered as so "expendable", that they can sell them "even" in DRM-free store (basing on the rule "we can't lose on those titles anymore, so might as well throw them to someone "outside". Plus the facts that GOG is adjusting old games to modern systems and have some faithful fanbase (group who is willing to pay for those games and is numerous enough to make profit).