Note exact time of aircraft. Telephone Gatwick Airport - Freephone 0800 393 070 or email: noise.line@gatwickairport.com
Ask for written reply. If appropriate, ask for a print out of the aircraft track, and details of its height over your house.
If told aircraft did not exceed noise limits, point out that the monitors do not detect all infringements because not all aircraft fly over them. Nor, probably, are the monitors near your house (they are about a mile from the end of the runway).
If told that the matter will be taken up with the airline, or with Air Traffic Control, or with some airport committee, insist on seeing response or action from that body.
If still dissatisfied, write to your MP. Copy to GACC please.