If your ticket is marked 'Southern Only', then you might* encounter some problems (as detailed in previous threads) as some claim that 'Southern Only' means trains with the Southern brand, however the NRCoC states that some tickets restrict you to a particular Train Company. All Class 442 trains are operated by the Southern Train Company. However some people believe that the brand Gatwick Express, which ceased to be a Train Company in late 2008, counts as a Train Company for the purpose of tickets. That is not a view I share.
* But probably not, as there are not on-board ticket inspectors on these trains, the on-board ticket inspectors appear to have been re-deployed to gatelines at Gatwick and Victoria.