Does a passenger aircraft have a gearshift?

A jet of traffic and a four-seat Cessna do not have it, because you can only give gas/grow or take away.

But, for example, a turboprop transport aircraft or military transport aircraft has a stepless “speed-connection”, the fully adjustable Constant-Speed propeller.

In this case, “constant speed” does not mean the flight speed, but the propeller speed. The stepless, electrically or hydraulically actuated adjustment of the propeller blade position allows the gas turbine to always run in the optimum rotational speed range or to transport a lot or little air to the rear according to the speed.

In the stand at the start of the start run, the propeller blades are in a flat position, comparable to the lowest gear in a bicycle or a car, with increasing speed is “upshifted” by increasing the blade position.

In the climbing flight, the propeller blade position is still in a relatively flat position due to the low flight speed, and only at a higher speed at the travel flight altitude, the propeller blade position moves to a steep position.

The great bonus of the propeller blade adjustment is the enormous braking effect which can be achieved after the landing. This is clearly larger than that which is achieved, for example, with the thrust reversal of a jet.

A special case is when the braking effect due to the propeller blade position is already used in the air when

• a very steep descent is necessary in the mountains through the terrain contour
• in war areas, by as short as possible a steep approach to the airfield, the exposure to hostile fire should be kept as low as possible

The latter approach technique has become known under the name “Sarajevo approach” after the former field of use of the German Bundeswehr during the Jugoslavia war, see here with the former Turboprop transporter C-160 Transall:

https://www.youtube.com/watch?v=8xkHOb17-48&t=36s