Gatwick to York

You have to go from Kings Cross. Our rail system is complicated, but to make a long story short if you inputted Gatwick to York on the VTEC website then it should have bought you connecting tickets between the stations. You should have a valid ticket from Gatwick to Victoria, you'll have to buy a separate Tube ticket from Victoria to Kings Cross, and then the first ticket will be used again for Kings Cross to York.

Tbh I think the bigger problem you have is the very tight connection you've given yourself to get from Sussex to North London.

I went to the VTEC website My Account and looked at the details for my trip, and it says depart Gatwick at 11:47, arrive at London St. Pancras 12:48, walk from London St. Pancras to London Kings Cross, depart London Kings Cross 13:30, arrive York 15:32.

Does this mean getting from Gatwick to London Kings Cross is included in the cost of my ticket (50 pounds)? Is the schedule doable? I checked google maps, and it appears London St. Pancras and London Kings Cross are very close.

Thanks.

Jesse, yes there is a station at Gatwick airport, at the south terminal. It is on a main line and has trains to many destinations. If you arrive at the north terminal you need to get the people mover thing to the south terminal, and follow signs.

The Express train from Gatwick to London is called the "Gatwick Express" and runs to London Victoria station, from which you could take the tube to King's Cross.
But, I recommend you DO NOT get that train. Instead get a normal commuter train with a destination of Bedford. This will take you direct to Kings Cross/St Pancras without having to use the tube (carrying luggage up and down escalators). It will have intermediate stops and be ~5 minutes slower than Gatwick Express+tube, but will be an easier route.
Ask one of the uniformed staff at Gatwick station, tell them you want a "Thameslink" train to St Pancras, and they will direct you.
From St Pancras you cross a road to get to King's Cross.

But, I think your timing is tight. Gatwick to Kings Cross will take approx. 1 hour by either route. Taxiing, disembarking, walking through corridors, immigration etc. normally takes about 1 hour. You only have 2h20 in total. This leaves you no contingency if immigration is slow or your flight is late.

CORRECTION. Jesse, I've just seen you second post, it is 11:47 departure from Gatwick, sigh of relief!
Yes, you have a ticket that covers both trains, (1) Gatwick to London St Pancras, and (2) London Kings Cross to York.
And that 11:47 is a train to St Pancras, so you have to walk across the road to Kings Cross, and you have 40 minutes to catch your second train at 13:30.

Trains from Gatwick to St Pancras are every 15 minutes, there are no reservations (commuter train), so you can get an earlier train if you arrive earlier at the station.

If you are jetlagged you could possibly be confused by all the trains. You will see Gatwick Express and Southern as well as yours which is Thameslink. The destination for the Southern will be London Victoria (and other non-London destinations), and the Gatwick Express will be London Victoria too.

The Thameslink will show destinations of both Brighton (and possibly Three Bridges) in the opposite direction and Bedford for the train you want. The route to Bedford and St Pancras International is completely different to that used into Victoria, so you can't get on the wrong one and expect to still find your destination without lots of problem. The departure screen will show the operator name, the departure time, any delay, and the stations to be called at. One station in central London that you will go through is Blackfriars. That station is on top of the Thames River, a bridge for the whole distance, made of glass. Beautiful views.

The Southern and Gatwick Express both go into a terminal station so the London stop is the final stop and there is plenty of time to get off. With your case on the Thameslink the stop is not long - a matter of 30 seconds to a minute - so you and your luggage and and all people in your party need to be ready to hustle off when you arrive at St Pancras International.

To help you be prepared I'll list the stations you will be passing through after Blackfriars. The first on a Friday will be City Thameslink (my fav, I was there yesterday), then Farringdon where the train changes electric supply from third rail to 25,000 volt overhead and the train stops for a few moments while that happens, and the next one is St Pancras International. When you get off there is a lift, and there are double banks of escalators. Kings Cross is just across the road.

Just to reiterate - while you MUST take the specified train from Kings Cross (it sounds like you've bought an Advance ticket which is for a specific train and comes with a seat reservation), feel free to take an earlier train from Gatwick to St Pancras as long as it's the Thameslink train northbound (the destination should be Bedford). That would give you a little more time to get your bearings at Kings Cross, although it is literally a five-minute walk from St Pancras.

Once at Kings Cross, the platform number for your York train won't be announced until about 10 or 15 minutes before departure so plenty of time for a coffee or lunch. I'd also strongly suggest buying any food you want for the journey at the train station as there's a far wider and better variety than you'll find on the train itself.