So, i've read that due to the amount of content in Rayman Forever, a great part of the soundtrack has been cut out from this version of the game.

Is it possible to put all the original PSX musics on this Rayman Forever from GOG?
If it isn't possible, i do prefer to emulate the PSX version...
If it is, could you please explain how to do it?

Thanks a lot.

You can find the soundtrack here:
https://raymanpc.com/wiki/en/Technical_information#Missing_soundtrack

The PS1 exclusive MIDI soundtrack can however not be used.

Hmm... Do you think is worth buying this version of GOG?

Well, that's entirely up to you. If you want to play the original Rayman game, then this version is great. Some may consider the PS1 version the best version as it's the most complete, but I personally prefer the PC version. The rest of the ports are mostly downgrades. If the music is throwing you of, it's not really a big deal at all. If you replace the tracks with the ones I linked it'll be a complete soundtrack in the game playing on all stages.