Maybe on a direct road travel time comparison. However you save much more than the time difference you quote, getting from the plane, through the terminal and picking up your ride at Gatwick, by using Southend.
Airport transit times might be one thing but so might be Dartford Crossing times ... never mind the toll.
As far as south of the river is concerend, unless within the immediate vicinity of south of the river, SEN is always going to be on a loser so best concentrate on north of the river and the A127 link to the M25 then the M11, M10, A1M, M1 etc.