You may wish to consider looking at a a railcard - either a Network card, or one of the others - be it 16-25, Family+Friends, etc...
In particular, you should note that the Gatwick Express is operated formally by Southern, and that you may be able to claim that tickets that are not purely Victoria-Gatwick should permit you to travel if they say "Southern only".
Bit of a grey area really - some ticket inspectors will allow this, while others won't. If you're gonna try this, get an email from some official rail body confirming it's valid beforehand.
If you want to get to London quickly without paying a fortune, consider instead the trains to London Bridge, which are actually almost as fast as the Gatwick Express, rather cheaper, and definitely do allow the standard railcard discounts.