Southern and Gatwick Express ARE the same company, but as GX is faster [all two minutes of it!], that justifies the separate brand and extra six quid you get fleeced of.
Except, as we all know from the ASA telling them off, GatEx isn't the fastest way from Gatwick Airport to central London. That goes to First Capital Connect, at 28 minutes to London Bridge.
In all fairness to Southern / gat ex the FCC time doesnt include the standard 5 minute waiting time outside London Bridge (or the late arrival there) fortunately theres loads of padding through central london to be back on time at St Pancras. Also London Bridge is hardly central London its Southwark and I doubt many foreign tourists will be staying at hotels in that locality.