The Stim-gas grenade is a new piece of equipment in Judgment and is used to regenerate health for your squad. The health regeneration effect is very quick and can restore a player to perfect health almost instantly. During the campaign, they don't get used as frequently as they do in multiplayer because you can generally control the tempo of combat pretty well so regular Frags are more useful.
In multiplayer, the Stim-gas grenade is the standard grenade type of the Medic and replenishes automatically. In multiplayer it is particularly useful. Keep in mind that it is more important to cooperate with your team than to just support it. If your team works well together then it is possible your natural healing is sufficient and that the Medic is a wasted slot.
