The story
So called because the vehicle creeps in the mechanical sense of that word: it moves very slowly, such that a person on foot can outwalk it, for example.
Etymology from Wiktionary, CC BY-SA 4.0.
creeper gear
Today it means: The lowest gear on a vehicle: especially, a dedicated super-low gear, available in some trucks and tractors, that is even lower than the usual low gear, which is labeled as first gear. Full definition →