This is the Great Planes ElectriFly 24mm Ammo Gear Drive.

For 24mm Ammo Motors and most other 24mm In-runner motors High-precision spur and pinion gear for quiet operation. Dual ball bearing-supported output shaft Bolts directly to the ElectriFly Brushless Motor Mount (GPMG1250). All metal construction for added durability.

Specifications :
Length : 2.29 in. (58.2mm) approx.
Width : 1.32 in. (33.4mm) approx.
Motor output shaft dimension : 3mm
Prop adapter shaft dimension : 4mm to 1/4-28
Gear ratios included :
2.5:1, 3.1:1, 3.6:1, 4.5:1
2.5:1 with 16T pinion and 40T spur
3.1:1 with 16T pinion and 50T spur
3.6:1 with 11T pinion and 40T spur
4.5:1 with 11T pinion and 50T spur
Weight : .75oz (22g)