(2-part post):
AAGH!
I was assuming that you'd got plenty of time!!!
Firstly, since your question refers to Kings Cross, I assume that you'll be arriving there by train. You can book tickets from your starting point, with 'Gatwick Airport' as the destination. If you do so, your tickets will cover both legs of your National Rail travel and the underground connection (if required) across London.
If you've already bought tickets to Kings Cross (and can't change them for tickets to Gatwick Airport), I suggest getting tickets, before you travel, from Kings Cross to Gatwick Airport. (Your local station can sell you these). Ticket queues at London mainline stations are often 10 or 15 minutes long, so you shouldn't wait until you get to London before buying tickets.
Gatwick Airport is quite a long way from central London. (About 28 miles). The fastest train service is the non-stop Gatwick Express. It runs every 15 minutes, from Victoria, with a journey time of 30 minutes.
To get from Kings Cross to Victoria, you should follow the signs for 'Victoria Line' (Southbound). Trains run every few minutes, and the journey time is around 10 minutes.
So that's (say) 10 minutes to find your way to the Tube platform and wait for a train, 10 minutes on the tube, 5 minutes to find the platform at Victoria, perhaps 10 minutes to wait for a train and a 30 minute journey. That's 65 minutes to get onto the platform at Gatwick Airport station. Then you've got to get from there to the check-in desk. (Ouch! That's tight!!!).
(Incidentally, there are other - slower - trains from Victoria to Gatwick Airport. If you decide to travel via Victoria, make sure you take the Gatwick Express and not the Southern services).