The following is the spiel on
Monarch's website w.r.t. when a gate closes before a flight:
What time do I need to arrive at the departure gate?
Please make sure you arrive at the departure gate at least 45 minutes before departure. Gates close 45 minutes prior to departure and we reserve the right to deny boarding and cancel the reservation of any passenger arriving after this deadline. Please allow plenty of time to get to the departure gate, bearing in mind that security control can get very busy at peak times and some departure gates are quite a distance from check-in.
I don't know how they operate in practice though.