The same basic definition of a gatekeeper, except these control the time speech is made and not the speech itself. Time and space in a newspaper are similar. Therefor, gatekeepers of time can also be gatekeepers of space.
The television station not only gate keeps the information, but they are also the "gatekeepers of time." Or, the sales department at the radio station sells drive time at a rate I can not afford for my speech. Therefor, I must speak at a time not of my choosing.
In the case of the print medium, gatekeepers of space control "where" and "what size" my speech may take up, if any. Cost is also a factor in the space time equation of time/space gatekeeping.