Gatwick to Parliament Square area

£19.90 walk up Anytime Single at Gatwick Express, £16.20 walk up Anytime Single on Southern.

Southern takes anywhere from 1 to 4 minutes longer to make the journey.

Seating 2+2 standard on Gatwick Express, 2+2 or 2+3 (depends) on Southern.

Under the skin the company running them is the same - Go Ahead.

Fewer non-airport locals on the Gatwick Express.

First time to Gatwick.

If you are in the North Terminal take the free cable-car propelled people mover between terminals to the South Terminal and then just turn following signs to the station (queues for machines and people common). Unfortunately Southern doesn't use e-tickets. Even if you buy tickets online from them you have to queue at a machine to collect them (sorry).

Escalators and lifts as well as stairs down to the platform. Platforms are narrow and wind-swept. Nice views heading into London. If you sit near the front of the train (everybody else in the know will want to too) you will be some of the first through the barriers (hold onto that ticket!! You need it!!!) as Victoria is a dead-end station. If you arrive on Gatwick Express you will be at the far left of the station, and there is a bit of a wide corridor to navigate to reach the main concourse (toilets on the left near the medical centre) and the tube and buses straight ahead and just to the right. If you arrive on Southern you will probably be towards the left, but could be as far right as the centre. Same exit to buses and Tube, pretty much straight ahead. There are also exits to the left and right.

Thanks, Nigel. I am glad I asked! I neglected to explore the " other fares" on Southern and see the walk-up Anytime rate. Lesson learned.

Bus to Parliament Square

The 24 and 11 are good choices between Victoria and Parliament Square.

http://content.tfl.gov.uk/bus-route-maps/victoria-a4-150717.pdf

gives you a spider map of Victoria. ((NOTE: TfL no longer publishes paper Central London Bus Guide maps, only these spider maps and tube maps - because everything including routes changes so often)

Number 11 doesn't go through the bus bays outside the station, rather it is from stop "S" on the road.

Number 24 is just as much fun to find. Out the right hand side door by platform 1 to stop "J".

It is all on the spider map. Use the key in the upper right edge.

We would feel right at home on the 11. We rode it down Kings Road to Worlds' End for a nice dinner at an Indian restaurant, sitting in the front seats upstairs.

Gatwick Express is THE way to go IMHO. That said, I don't know when you're traveling but there is going to be repair and construction going on in December and January...here is the info about it in case you might be affected. We are going to be there and I just noticed this yesterday when I was looking at tickets. Only a few days of complete closures but train service will be reduced.
https://www.gatwickexpress.com/travel-information/plan-your-journey/christmas-and-new-year-2017

@anita.
That info is for last year (2017)!
I have no doubt ther will be work going on and closures between Christmas and the New Year, there always are. But the page you linked to is not this years info.