janni.. I expect you'll get plenty of answers but what my wife and I do when we are going spotting to somewhere unfamiliar is check on an OS map to see where one can see the approaches to the runways(s). This is particularly useful for Heathrow where it is very difficult to do spotting from within the airport.
It's yonks since we visited Gatwick, but a quick look at the OS map shows a road running right round the SW side of the airfield from Lowfield Heath to Charlwood. Maybe that would afford views? Check these links:
http://www.gatwick-airport-uk.info/g...nespotting.htm
http://www.gatwickaviationsociety.or...troduction.asp