List of all destinations flown from Gatwick, and airlines, here: https://en.wikipedia.org/wiki/Gatwick_Airport#Airlines_and_destinations
Easyjet is the only airline flying from Gatwick to Valencia.
Prices only ever go up. Buy today, if not sooner, direct from Easyjet: https://www.easyjet.com/en/
This is peak holiday time, flights fill up early. For the week between Christmas and the new year they are already well over £100. For the first week in Jan they are lower, £25-£80.
Are you intending to fly into Gatwick and out again on the same day?
There is a big risk in doing that. If the first flight is late and you miss the second, you will have to buy a new ticket, at maximum fare, and possibly stay in a hotel overnight. Easyjet has no refunds for missed flights, and a strict deadline of 40 minutes to be at check-in. I you miss the flight they will not wait.