The Voodoo Gnaar is a scrapped Serious Sam 2 weapon.
The Voodoo Gnaar is a mysterious weapon. The only remnant of the Voodoo Gnaar is a HUD image for it. Its style seems to suggest that it was a leftover from a much earlier version of the game, which explains why there's no other parts of it found in the game's files.
Based on it's HUD image, it seems that it was a voodoo doll of a Gnaar with pins on and near it. Anything else is just pure speculation.
Since there's no way to determine how it worked, there's no way to conclude how it got scrapped.