The operation of trains between London and Gatwick is confusing, and Southern do their best to keep it that way. The cynical might think confusion allows them to encourage unsuspecting tourists to pay expensive "Gatwick Express" fares without realising that cheaper alternatives are readily available.

All trains between Gatwick and Victoria are operated by SouthernTrains. Some trains are branded "express" - but they are still operated by Southern.
If you have a through ticket from Gatwick to north of London, you can use any train service into London. You could also use the FCC "thameslink" service if that suits your journey better - though on the date you're travelling there are no cross-London through services to St Pancras.