Yes, unfortunately, current frequencies make it a bit harder than usual to organise day trips (assuming you do not want to transform this into a proper overnight/weekend trip away). If you merely want to "buy your way out" of quarantine, airports allowing back to back might be an option, but I'd double check with regulars that transit is not made harder than usual at the moment and that back to back is still possible as many countries may have channeled people differently during the pandemic.

Otherwise, I can always recommend Nice. There are two BA flights a day at the moment (one of the few destinations to get them) and also Easyjet from Gatwick, and you are a direct €1.50 and 25 minute tramway ride away from the harbour for a nice meal and stroll by the sea or beach break or Fenocchio ice cream in the old town before returning to the airport in due time. Obviously even better with an overnight stay. I'm sure that there are a few other good options, but obviously much fewer than usual due to reduced frequencies.