Miaghstir: Ubisoft owns Myst 3 and 4, Cyan owns the other games and the brand.
Edit: ah, ninja'd by over half a day. I blame a tiny phone screen making it annoying to read whole threads.
Yah, reading GOG on a phone is a pain in the ass. Wouldn't be a bad idea to create a version of the site designed for phones.
GOG already has both Ubisoft and Cyan Worlds signed on, but releases from them tend to be fairly inconsistent and sporadic, so while GOG's policy of staying mumm in its dealings with publishers and with prospective releases means that real background information is generally hard to come by, I think we can safely read between the lines and assume that GOG's dealings with them have been difficult.
For example, we have PoP Sands of Time, but still no Warrior Within or Two Thrones. We have Settlers 2 and Heritage of Kings, but no Settlers 1, 3 or 4.
I thought Ubi had the rights to Myst 5 as well, but it would appear I was wrong.