There's no question about Gatwick being a "London Airport".
The modern day Gatwick was originally built in the late 1950's (opened in 1958) specifically as London's 2nd airport, replacing several older, smaller London area airports.
It replaced the last remnants of Croydon airport, which had already run down to almost nothing after WW2, as well as Blackbushe and Bovingdon airports.
Most of Northolt's substantial commercial traffic, moved to Heathrow in the mid 50's.
The new Gatwick, was built mostly on the old racecourse and farmland to the north and west of the original 1930's airport of the same name.
The original 1930's "Beehive" terminal building, still stands in the middle of an industrial and business area, located half a mile to the south of the eastern end of the present day Gatwick runway.
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