Gatwick to Stansted airport transfer

The fastest way to travel between Gatwick and Stansted airports, is the Gatwick - Stansted train option via Central London.
But you could also take a shuttle bus between the two airports if you have 3 hours. https://www.londontoolkit.com/travel/gatwick_stansted_bus.htm
Often handling baggage is easier on a bus vs. lugging luggage through stations and on trains.

Even with 6 hours, most British would recommend against changing between two London airports. Too risky, too much time wasted.
Is there any way you can change your tickets to use the same airport?

If you insist on doing this, you can look up train times on National Rail: https://www.nationalrail.co.uk/

• De-planing and immigration at Gatwick, getting to train: 1 hour on a good day
• Train Gatwick - Victoria - (tube) - Tottenham Hale - Stansted: 2 hours, but you may have to wait up to 15 minutes for your train
• Check-in time at Stansted: 1½ to 3 hours, depending on where you are flying to

Plus your incoming flight could be late.
CONCLUSION: You have no free time.

Is your 6 hours from nominal arrival time to nominal departure time, or when you say you have 6 hours is that after you have allowed 90 minutes at Gatwick and allowed 90-120 minutes (likely, your airline will know for sure) for check in at Stansted? In other words, do you actually have 9 or 9 1/2 hours between arrival and departure?

This is two unconnected tickets, right? Ryan Air or Easyjet or something else at Stansted?

I have 7 hours total. I figured it would take an hour to get through customs.

I have 7 hours total. I figured it would take an hour to get through customs.

• It takes 1 hour on a good day to get through immigration (de-plane + walk through endless corridors + immigration + get to Gatwick station, which if you land at the North terminal requires a shuttle train).
• On a bad day can be 90 minutes+
• And where are you coming from and going to?
  • If you are arriving transatlantic, planes can be +/- 1 hour depending on wind.
  • if you are leaving transatlantic, then check-in time can be 3 hours instead of 90 minutes.

Still tight on a bad day!

I'm going to Bordeaux, France.

Assuming you land on time etc, I would use my spare hour to exit at Victoria train station, walk 15 mins to Buckingham Palace and grab a sandwich at one of the many places you will pass on the way. Then continue on your journey. There is nothing touristy around Tottenham Hale.

Thanks for the suggestion! It's going to be tight, and I might be exhausted, but I don't want to be kicking myself waiting in the airport for 4 hours if the second flight is delayed, so better have a second option ready at hand.

I know where you are going but not with which airline.

With some if the flight will be delayed you can check in late. With others, such as Ryanair and others you need to check in before the official deadline regardless of how late you know the flight will be or you won't be allowed on the flight.

It is worth checking if that would apply to you.

Thanks for the tip!