The term is mostly used in radar, where range gates are used to select certain targets for further processing.
A range gate would then filter out all the other targets that might be visible to the radar.
The return within that gate was then automatically tracked without further operator intervention.
In weather radar, it is common to have a series of continual range gates that separate out returns at different distances and then process them to extract Doppler shift to measure wind speed.
In these cases, it is common to refer to each gate as a range bin.