Hi Carol
You can get a train direct to Gatwick from either London Bridge or Victoria. From London Bridge it takes between 30mins to an hour, depending on whether you catch an all stations service or the once that just stops at East Croyden. You don't have to change.
From Victoria you have the choice of the Gatwick Express - more expensive but non stop in 30 mins or a Southern Train. Again, these take between 30-50 mins depending on if you get a stopper and go from platforms 15/16.
You'll need to find your way from Kings Cross to either station by bus or taxi if you don't use the tube. I'm afraid I can't tell you which one, but if you have a look at the 'Transport for London' website, you should find the details on there.
Have a great trip
Mandy