however I can only use the opengl32 mode

But that's exactly what you'd expect if you were using a 3Dfx miniGL driver.

If it's Quake 3 based, then you might want to get WickedGL – but again, if I'm not mistaken, this just provides a replacement opengl32.dll.
http://www.3dfxzone.it/dir/3dfx/wickedgl/index.php

Yes, but the game seems to use the main gpu (wich is an ATI rage pro) instead of my voodoo 2 sli. Also, the game doesn't come with any dll files, so I can't replace a non existent file ... The game seems to look for that file in the Windows directory, so if I replace that file it will screw up everything, isn't it ?

The game seems to look for that file in the Windows directory

In cases like this, the general procedure is to copy 3DFXVGL.DLL (I think it's is in system32, usually) to the same directory as the game's executable, rename it to opengl32.dll, and then run the game. The game checks the current directory first, you see.

Anyway, I think the WickedGL installation should take care of all that for you.

Copying and renaming the file worked ! Thank you very munch 😀 !

Now I can enjoy this game ... at ~20-30FPS most of the time 🤣

it won't work as good though because of the stripping of a 3dfx gamma extension.