Yep,

what are the diferences between GOG version and the DVD original one ?

Well the original was actually on a CD. I believe that the main difference is that for the GOG version, some bugs related to very fast processors were fixed. So if you tried to run the original CD version on a modern-day machine, certain things would not work, like trying to ride a Twon-ha (the Twon-ha would not move). I experienced this myself when running the original CD version a year ago... I had to use a separate program to slow down my CPU to get the game to work. With the GOG version, this will not happen.

I think that is the main difference... but I actually haven't yet played my GOG version. Can anyone else confirm this? Or if there are any other differences?

There is also an unofficial patch to run the game in high resolutions, and this patch may only work on the GOG version, rather than the original CD version. This is NOT provided by GOG, but you can find the patch by looking through the GOG forums.

To be clear, the GOG version will run at the original resolution, unless you add the fan-made patch.

That's not entirely accurate. Dolby Digital is used in extras, not in the game (which relied on EAX and A3D). Also the music was redbook (a.k.a. standard CD audio) on the CD ROM version, whereas DVD version used 128 kbit MP3s, which were rather poorly ripper with some extra snap, crackle & pop. Since GOG's version had the same MP3s, it's based on the DVD version.

DVD did, however, include all previous patches. The CD version wouldn't even install without a patch (which featured the installer) if DirectX was newer than 7.0. That caused DX version check to end in a critical error to InstallShield, which resulted to a 'panic uninstall'. One way to avoid it was to shut down the system ASAP when DX version check started. For some games (e.g. East Front II) this was the official workaround. Thankfully only a few games had that bug-ridden version of IntallShield.

Those indeed were the days. :)