If you want mouselook, I would recommend using WinROTTGL. It is a sourceport, and even if it does not reach the level of the ones available for other games (zDoom and eduke32 for example), it is a nice way to play the game. If you are playing vanilla DOS version, I would advise to forget about vertical aiming (the game itself autoaims up when a foe is way up).
If you want to try the sourceport, you must go
here. I downloaded it not a week ago and worked quite nicely, even if the loading times increased a little.
I downloaded
, [url=http://www.riseofthetriad.dk/GLgraphics.zip]this,
and [url=http://download.dll-files.com/a01900aa57d09b4db5e63c5b8a787c06/glut32.zip?0XMlQIZFkX]this.
The second link are graphics, so I
think it is no mandatory, although the textures are like the old ones, just higher. The last link is a .dll I had to manually look for, do not know why.
For installing, create a new folder and extract all those .zip files in the posted order (just to make sure). Then copy your .rtc .rtl and .wad files on your GOG install dir and you should be fine.
The manual aiming with this sourceport is
way far from perfect, but I think it is worth a look at least.