In message , Peter Lawrence
writes
My mapping software says it's 45 minutes on a clear road. We don't yet
know what day, or time of day, the OP will be travelling.

Guess he's arriving in the early morning, as transatlantic flights
tend to.

At Gatwick, that's probably the case. However, there are late arriving
transatlantic flights at Heathrow, which means it's always best to ask.

You don't get a clear road then!!

You would at the weekend.

IMO the timing is very tight.

Everyone agrees that!

Nobody has mentioned the direct bus which, in spite of the traffic, is
probably the quickest, see
http://www.nationalexpress.com/p.cfm?n=pai-htg

Terry has. The problem being that you need to wait for the next one, and
apparently the journey Gatwick South to Heathrow T1-3 is noticeably
longer than Gatwick North to Heathrow T4.
--
Roland Perry