Sooner. 1852.

Airplane compasses and torpedoes used gyroscopes in WWI.

Does it belong in the steering/control systems of a model - no (IMHO). The dihedral of a trainer does the same for roll and the throttle can trim a properly balanced model for horizontal pitch. You actually need three gyroscopes to control the yaw, pitch and roll.

A gyroscope will not keep you airborne, but will resist orientation changes - even if those are needed corrections. A gyroscope initialized with the aircraft's nose pointed into the ground at a 20º angle will happily keep the model heading into the ground at a 20º angle.