GIBBIN3 is a modification for Quake improved by Jason Carter (the original was solidmon). This modification makes it so that all Monsters have solid bodies (Ranger does not as the author played Singleplayer more than Deathmatch), which can be shot at to Gib them. All Monsters can be walked on top of (this is the most signifigant update, solidmon was limited in the fact that all Monsters would have bounding boxes that had to be physically jumped over) and can stack on top of one another. Monster corpses have a smaller amount of health than the normal Monster, meaning it is easier to break them apart. Every Monster has a unique Gib combination, beyond Spawns and Rotfish, which do not Gib in the vanilla game and thus have not been changed. Note also that Fiends do not have solid corpses.
Note there is a glitch in this mod; a Scrag's corpse counts as another Monster, resulting in counters that can go over the Monster counter at the end of each level.
As the monster head spawns directly below the Monster, this can become a problem if the player is standing on top of the dead Monster and hits it. The head will spawn directly beneath the player, resulting the the player being unable to move outside of the bounding box. Furthermore, since the heads are not solid entities, this means there is no way to get off the heads beyond using Noclip.