Kings Cross to Gatwick

Looks like there are 3 direct trains every hour from St Pancras Int'l across the street. Takes about 1 hour.

You can take a train from St. Pancras station (across the street from King's Cross) to Gatwick. Rick explains in his England guidebook:
A train also runs between Gatwick South and St. Pancras International Station (£10, 3-5/hour, 45-60 minutes, www.firstcapitalconnect.co.uk)--useful for travelers taking the Eurostar train (to Paris or Brussels) or staying in the St. Pancras/King’s Cross neighborhood.

Thank you for your replies. Another question for you, @Tom_MN and @Casey, would it be easier/quicker to fly out of Heathrow? It looks like we can get a British Airlines flight to Rome from London from either airport. Which of the 2 has easier access via public transportation? Thank you.

Thameslink provide a decent service to Gatwick. It has a few stops in The City after St Pancras, including the incredible glass bridge over the Thames station at Blackfriars. Look to the left as the train is in the station and see St Pauls to the left, and Tower Bridge perfectly framed in the glass. A farewell photo moment if I ever saw one.

London Bridge station is being rebuilt from the inside out and while that is happening the trains take a somewhat scenic diversion which slows them down a touch, but they pretty much stick to the timetable.

The train goes straight to the South Terminal at Gatwick. If your flight is from North Terminal there is a convenient free fast and scenic people mover next to the station.

Gatwick is compact and only 2 terminals. Heathrow is a bit more spread out with terminals here there and everywhere.

Lifts and escalators at both ends of the Thameslink trip mean step free.

I'm afraid that the Rick Steves information quoted by Casey is quite out of date.

Nigel, thank you very much!

If you pick Heathrow there is a direct but slow service from Kings Cross on the Piccadilly Line tube. It will probably take much the same time as the train to Gatwick or a bit less. The fares may or may not be cheaper depending on whether you have an Oyster card. Like the Thameslink it is now officially step-free, although the elevator at Kings Cross occasionally breaks down and is inconveniently located - you need to use the entrance at the far end of St Pancras railway station.