I would put the full route into one of the route planner systems, and then factor in an extra hour to an hour a half to allow for blockages. Bit of a bad time really as 7 is when the morning rush hour begins.
SW London (top of M3 at start of A316) takes 45 mins to Gatwick at that time. The problem will be the part of the M25 around Heathrow which gets clogged up. You`ll need to factor extra time in for that.
Adik hasnt actually said if he is taking a flight, or if he has anything more than hand luggage. I have done the Heathrow to Gatwick by car thing so many times and it has taken anything from 45 minutes to 3 plus hours!!!!
Most of the M25 is 50mph. 1hr on M25 & 30mins from exit to Gatwick @ 70mph.
If you're near Stanstead the Gatwick express coach (every 10mins) is quickest via shortcuts, tho' expensive if you're travelling with company but you'd save parking charges.
Never think that it will take you an hour from Heathrow to Gatwick Adik, It COULD take you an hour to move a mile on that car park! If you have a plane to catch, stop at the Premier Inn, that way you are there ready for your flight without getting wound up before flying.
i work next to gatwick airport, it does get very busy as the manor royal industrial estate is right next to it so hundreds of people are getting to work. i would leave within plenty of time.
I know TWR. My mother hated the M25, and it was so ironic that at her funeral the cortege went on to it for some of the way. All the flowers were flying off the top. It is the only time in my life I ever saw funeral traffic on the motorway, though its normal speeds are quite suitable for the purpose!