This is an area effect garlic spray, basically it sprays garlic in a 5yd radius. If you throw the grenade then you must get 3 Success Levels to get it to land where you want it to go on a Dexterity + Getting Medieval or possibly a sport roll for those who play cricket or rounders (Americans call rounders baseball, btw). Any less success levels and the grenade doesn't exactly go where you wish missing some or all your targets. No success levels and the Garlic Grenade falls somewhere totally wrong and probably embarrass' the thrower.
Garlic Grenades inflict a -2 penalty on all vampires in range (just like the Garlic Spray) Humans and demons that are not effected by garlic, probably just give the attacker a bemused look.