When flying longhaul, we usually fly Business Class (BA/Virgin/Continental). If we fly from Gatwick, then we always stay in a hotel overnight before we fly. We live about 2.5 hours from Gatwick, BUT there is ALWAYS something icky on the Southern part of the M25... so it always takes longer to get there.
As Andrew you will want to be there at about 9am (for an 11.15 flight), maybe a little earlier (since you need to park the car, wait for a bus, then get to the airport - unless you do "meet and greet" parking, of course... then 9am would be fine).
I don't know how long it takes you to get to Gatwick, but you would need some contingency in your time (to allow for problems). If you are going on a weekend, it shouldn't be too bad (a weekday would be a nightmare - rush hour...).
Also, depending on how long you are going for, you can get some very reasonable hotel/parking packages (look at holidayextras.co.uk) for really not much more at all than just the parking (£20/£30 more).
Like Andrew, I find staying at Gatwick the night before a LOT less stressful way to start the holiday and it is quite exciting

. I would also rather stress about "have I got everything" at 7pm the night before rather than 5.30am the morning we fly...
Boo