After playing hours with the Rayman Optimum version I went back to the GOG version and unfortunately I have to say that the GOG version is not nearly as good. It begins with the wrong music, continues with the missing intro, languages and VSync. The Rayman Optimum version does even look better than the original and it is possible to play it in Full HD resolution (though it won't look like a Full HD program). Since the Rayman Optimum version is not official I don't think that there is much hope that GOG will some day add this version but it really is much much much better than the GOG version which I bought first and will not download anymore.
From what I think I understand about the situation, GOG's hands may be somewhat tied on what they can do and which version they can use. Optimum is a combination of various Rayman 1 releases into a single unofficial one, including as many user-friendly configurations as possible, GOG were likely given Forever as their only option, including all of its limitations. Apparently even the previously released Gold edition is better than Forever in terms of actual base-game completion; supposedly Ubisoft added creator videos to Forever that forced them to sacrifice some of the music, which is really silly.