Believe it or not I think you will find that you won't have a lot of time to go very far afield for lunch. Once you factor in the time spent getting off the plane at Gatwick, going through Immigration and picking up bags etc., the check in time at Stansted and the travelling time between the two, I reckon you'll have approximately an hour to spare (being optimistic). It is entirely possible that you will be held up a bit somewhere along the line and won't be able to get any spare time at all.
If you are lucky, then you might be able to have a quick lunch in a pub somewhere. This
website lists a couple of pubs in the vicinity of Victoria that do food: The Albert and The Grenadier. Alternatively you could go to
Dirty Dick's which is more or less across the street from Liverpool Street Station.
If you are very lucky and arrive early (it does sometimes happen when you fly eastwards) then a really good alternative would be to take a train to London Bridge, not Victoria, and wander down the road to
The George. When it's time to move on you can either take a bus to Liverpool Street or even walk (it apparently takes 24 mins to walk). This
map - if the link works properly - gives you an idea of the relative distance. This would give you the pleasure of eating in a genuine old coaching inn that was frequented by Dickens amongst others, and a great view of the city as you go over the Thames towards Liverpool Street. If you have the time, this is definitely going to be more exciting than going via Victoria.
Unfortunately, it's all too possible that you won't get any time to stop anywhere. In that case, I suggest you pop into
Pret a Manger (there's one in Liv. St. station) and load up with an excellent lunch you can eat at your leisure on board the Stansted Express.
The great thing about eating in London these days is that there's a lot of excellent food all over the place. Hope you enjoy your lunch.

Beebee