I mean I have that game on disk but if I can install, it is almost impossible to play it on modern systems.
I see many threads reporting issues; Is this GOG version somehow modified and fixed or is it basically the original binary ?

I also have the original disc of Messiah but can install without much effort on a industry standard Dell laptop.

Don't know much about the GOG version of Messiah.

And is it playable? I can install but then cannot play.

i can run the CD version in safe mode, but otherwise it crashes.
unfortunately in safe mode i have no sound and can't change the resolution, but other than that everything seems to work.
no clue what the issue is, though.

edit: i got it semi-working (can start it without it crashing), but i can't seem to get sound to work properly..
in D3D i get no audio at all. in Glide3x i get audio and music in the menu, but no music in-game, and no sound during videos.

This GOG version doesn't include the patch 02, that one has to be downloaded online. It's not necessary to play the game, but adds new graphics options (disable "pre-cached effects" to stop the game from crashing at a certain point).

It can be downloaded e.g. here: https://www.gameswelt.de/messiah/patch/messiah,117144