Gib death occurs when, after a character has been damaged, its health is less than the negative of its spawn health (or original hit points). In other words, the character has suffered, in total, more than twice the damage needed to kill it in the normal fashion. This may also cause the monster's body to be thrown very far from its actual location of death. Telefragging also results in gib death.
Only six characters may suffer gib death: the player, Trooper, Sergeant, Chaingunner, Wolfenstein SS, and Imp. A single special death sound, DSSLOP (a squishing sound), is used for all of these. Most other characters can be damaged sufficiently by a single attack to meet the above threshold, and almost anyone can be telefragged, but the normal death sequence is used in those cases.