Gibbing an enemy just means to cut it up into little pieces. Some of the weapons that count as gibbing are the Wall Knives, the Clockwork Mace, the Grinder and the Exploding Barrel. Most later levels have way more than 200 enemies, so this shouldn't be hard. Just get a decent setup with some wall knives (as they unlock the earliest) and if you're still worried about it, start using the exploding barrel. If you don't have it by the time you unlock the Swinging Mace, upgrade it and pair it with some tar traps. It is amazingly effective.