My guess is that a granny gear allows a slow enough speed at which a granny would feel comfortable driving.

Hear is a bicyclists meaning for a granny gear:

Granny gear n. The lowest gear available on a bike, which only a grandmother would need to use; designed for steep uphill climbing, but extremely easy to pedal in on flat ground."

Apparently, this term is used very frequently in the bicycle world (as it is in the old 4-speed truck transmission world).