Gatwick to Heathrow

There are direct buses between Gatwick and Heathrow, that would probably be the best way in my opinion.

This question is asked almost every week on this forum! Does “best” mean fewest changes, cheapest or most flexible? How many people and how much luggage?

The options are National Express coach, taxi, train into London then the underground and another train or underground to Heathrow.

Testing is available at Heathrow airport. Are you staying at Heathrow?

Jennifer, my apologies, I tried to search and didn't see this specific question. And this change (by airline) took us by surprise!
Transfer for 2 people with moderate luggage (1 carryon + 1 backpack each), but time is of the essence so we can do our C19 test and hopefully get out (maybe Paddington area - I've seen good recommendations on this forum) that evening.
So I guess convenience is a better criteria v. cost, etc. I will further research your options.
thanks!

And yes, we are staying at Heathrow. I'm wary to wait until last minute for testing as US requires it within 24 hours. I'm looking at the American Airlines/Verifly Qured.

As is often the case, the airport websites can be helpful. On this question, try https://www.heathrow.com/transport-and-directions/travel-between-airports

In 2020 we were going to Malaga Spain and had to transfer from Heathrow to Gatwick , booked the National Express coach thought it might cause issues but to our surprise it was very easy and efficient would do it again

The US testing requirement is "within one day", not "within 24 hours". So if you are flying home on Wednesday, for example, you can test at any time on Tuesday.

What time do you get into Gatwick? Gatwick has a walk through test center in the north terminal.

https://www.expresstest.co.uk/gatwick-airport-walk-through/#Gatwick%20Airport%20-%20Walk%20Through

In terms of getting to Heathrow there’s the bus option mentioned above. If you have time to spend in London you can take the train to Victoria station. Check your bags in the left luggage facility. Spend some time maybe get a nice dinner then hop the tube to Heathrow. There’s also a bus from Victoria to Heathrow.

If you intend to stay late do check the hours of the left luggage facility.

thank you for all the good suggestions!

I was very intrigued by the "within 1 day" comment - I think you are right! But then, our flight time was changed again so it doesn't matter as much. We are going with an online tele-health appointment (& bringing test along) via Qured, which is accepted by American. Fingers crossed!