how long to clear customs at Gatwick airport

There are lots of variables that you cannot control. First is on time arrival, deplaning, immigration can be fairly quick -45 mins or so, but dependent on the number of planes arriving at the same time. Can be longer. Customs is nothing - just a walk through the Green door - Nothing to declare. That is the easy one. Then find you way through the airport. As a min I would plan on two hours from scheduled arrival.

The train could be a lot faster. The airport express services have lots of room for bags. Then you can jump in a cab outside Victoria station if you would prefer not to take the Underground with your bags.

I haven't taken a cab from either airport in London, always a train or the Underground.

In London, the cabs are a nice way to get around in a hurry. I also really enjoy the bus services around London. You have a great view from the top deck of the bus, so even if it takes a little longer, it can be a nice way to travel.

I can't tell you about Gatwick, but I can say that I was in a line at Heathrow immigration on a Sunday morning for at least an hour. There must have been more than a hundred people in that line. It seemed like it was moving very slowly, but it could have just been normal for that sort of thing, or perhaps I was just anxious to get to my hotel.

Jennifer, You are asking the wrong question. Customs takes zero time, you just walk through the "nothing to declare" door, 30 seconds tops.
What takes time is taxiing, getting off the plane, a long walk through corridors and finally immigration (passport control), this will add up to at least 30 minutes, probably 1 hour.
And yes, you have summed up the pros and cons. Train, faster, cheaper and more comfortable. Car, slower, more expensive and more comforting to north Americans who don't understand trains.
I would vote you take the train to London Victoria, then if you are still feeling uncomfortable a taxi, otherwise the tube.
You cannot do your whole trip to London using taxis (you can, but it will be expensive). You will end up using the tube and buses.

Sandra and Chris - Thanks for the information.

Yes, we fully intend to utilize public transportation frequently. Just gathering information at this point!

I've flown into both Heathrow (LHR) and Gatwick (LGW). LHR from overseas flights, LGW for flights within Europe.

I've gotten thru LHR immigration/passport in as short as 10 min and as long as an hour. I believe the really short one was a flight that arrived about 7-7:30am. Same with LGW - I find it's usually in the 45 min range, but we are usually doing midday/early evening flights when more flights are arriving.

I remember once some friends were picking us up at LHR and we told them about 90 min after we were landing to pick us up. We ended up cooling our heels for about a half hour as we got thru rather quickly.

I flew into Heathrow a few weeks ago and had a very short (5 minute) line. Although, my flight came in about 6:00 AM and that could be the reason the line was so short. I would estimate 30-40 minutes just to be safe.

Is there a way to get a shared airport transfer? Those can be cheaper than private.

A good transfer company will ask for your incoming flight information so that they can be waiting for you at arrivals. I am sure they will not hold you to any estimate.

We just did this with two kids (4 year old and 8 month old).
Our flight landed around 10 and it took us a solid two hours to get off the plane, get through immigration, pick up luggage, find a bathroom and catch the train.
We also stayed in the Notting hill area. The Gatwick Express was so easy and the kids loved it after being cooped up for so long. We picked up our tube passes at Victpria Statoon as well