Three Descent titles—Descent, Descent II, and Descent III: Mercenary—were removed "effective immediately" from GOG.com on December 18 due to "changing hands and in-flux legal agreements" according to a statement by a GOG representative.
And now, according to a story at Kotaku with Descent co-creator Matt Toschlog and partner Mike Kulas, we know the reason. The Descent creators believes that publisher Interplay owes them "tens of thousands of dollars" in royalties and may be pursuing legal action against Interplay in response.
Toschlog says that the contract its company Parallax "signed with Interplay in 1994 was pretty typical" in that Parallax owns the copyright while Interplay had the right to publish the Descent games while paying them "a royalty on each copy sold." But Interplay hasn't paid them since 2007, which is when the whole dispute between Interplay and Bethesda started, though Interplay has given them "statements showing that royalties were due."
Until Parallax receives these royalties, it prevents Toschlog from creating a new Descent game since Interplay owns the Descent license: "We also made it clear to Interplay that before we signed a new agreement with them they would have to pay us all the money they owed."
Parallax is in the process of removing Descent from Steam too. Yowch.