Like Darvond said, it is. It's just not set up "exactly" like it probably should be/the same as it is on the likes of Steam. https://www.gog.com/forum/myst_series/myst_25th_anniversary
Essentially, all titles including Myst III & IV are available to purchase - the exception being no original realMyst release. GOG has them as a "buy series" option as opposed to Steam's bundle grouping as the "25th Anniversary Collection" (basically the same thing, just with or without a fancy title in your digital receipt).
More to the point, RealMyst was retired in favor of 25th Aniv, to which one can make opinions about in the forums for Myst.