Might I point out that the train from Euston to Manchester Piccadilly runs every 20 mins throughout the day and takes under 2h10. Stockport (close to Manchester airport) by train to Euston takes 1h55. From Euston to London Bridge takes 20 mins by tube and then another 30 mins by train to Gatwick.
Flying to Gatwick there is very little possibility to formally connect onto another flight.
Even if flying from Manchester to Sussex is a bit faster in terms of time, the train gives a less interrupted journey and a chance to do something productive.
Along with the increased landing charges, why would flyBE want to run a LGW-MAN route when they've recently cut LGW-ABZ ?