Gatwick and other busy airports have a runway hourly declared capacity which means airlines are allocated a scheduled arrival/departure runway slot (not to be confused with ATC flow control slots). If airlines operate on time in accordance with these runway slots then the number of flights offering to arrive or depart should be comfortably handled by ATC with delays kept to a minimum. If flights do not run to time then you often have a situation of too many offering to arrive or depart within a given time period and ATC just can't physically shift them on the runway quick enough without incurring longer than normal delays. So the ramp handling and terminal building processes must ensure an on time departure so the arrival stack or departure holding point does not get too congested.