The easiest of them would normally be Blackwater, which has direct tains from Gatwick once an hour normally at xx03. The only exception to that though is the 0910 which doesn't stop between Guildford and Reading. You could though catch this train, get off at Guildford and catch the next train which leaves about 20 minutes later, probably from the same platform, this would get you to Blackwater at 1035. If you were to catch this train you would need an off peak single (Via Gomshall) at £14.70 or £9.70 with a railcard. If you miss it there is a direct Gatwick to Blackwater train at 1003.
Otherwise you could go to Virginia Water via Clapham Junction every 15 minutes. Just catch any train to Clapham and change there into a Windsor or Reading train, and this would take about 1 hour 15 minutes. This would probably be the best idea if you get to Gatwick station at 0915 or something. The single on this would normally be £18 or £12 with a railcard, but whilst researching this I found out you could get an Anytime single to Blackwater (Route Clapham Junction) for £16.50 or £10.90 with railcard, which is valid via Virginia Water where you could stop.
They are your simple options.
Edit: Drats FenMan go there first. All I can add is the slightly cheaper ticket to Virginia then.
