As the name implies, "gain-scheduling" is to schedule the gains of a controller to account for changes in system dynamics that happen through state-space evolution due to disturbance or time-varying parameters. Such changes happen for example in an intercontinental ballistic missile, while center of gravity of missile travels along missile central line, because of fuel which is continually consumed and hence the mass of the missile continually decreases. To account for such a change in missile parameters, the controller should be scheduled for different phases of flight. It is simply like to tabulate the gains of a PID-controller for any specific stage associated with a process to be controlled.

To get acquaintance with the introductory idea, I refer you to read the introduction to the paper cited here:

https://www.researchgate.net/publication/260202232_Stability_Proof_of_Gain-Scheduling_Controller_for_Skid-to-Turn_Missile_Using_Kharitonov_Theorem

Although the essence of the paper is for flight control, however, it seems the introduction exemplifies some applications fathomable to a wide spectrum of scholarly readers.

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