From my experience the landslide to airside gates at LGW won’t let you through if your under 60 mins before the advertised flight time. However the staff at the gates can obviously override this if the flight is many hours late! If you get a nice member of staff at the gates they will check their screens and let you through this has happens to me at both north and south terminals! If however you get a jobs worth at the gates they will make you find an airline representative with the right authority found at the check in area from the respective airline to come and let you through! This has happened even when the plane isn’t even on the ground! Also the security at LGW shuts at midnight so if your flight is very delayed (until like 1:40 am as mine have been) so you must be through before then!