Done some digging, in the winter season 1991-92 which was Pan Am's last season at Gatwick and existence, the following schedule is listed - same flight numbers as previous Heathrow flights
PA54 DTW 19:35 LGW 07:55 A310 (no service Monday/Tuesday)
PA55 LGW 10:00 DTW 13:35 A310 (no service Tuesday/Wednesday)
PA98 MIA 18:35 LGW 08:10 747 (daily)
PA99 LGW 11:00 MIA 15:30 747 (daily)
Therefore the A310 and 747 where on the ground together for 2 hours
Also listed from Gatwick non-stop during this season is:
Atlanta Delta L1011 X2, British Airways DC-10
Baltimore TWA 767
Boston Northwest 747, Virgin 747
Cincinnati Delta L1011
Dallas Fort Worth American DC10 X2 British Airways DC10
Houston Continental DC10, British Airways DC10
Los Angeles Air New Zealand 747
Miami Virgin 747
Minneapolis Northwest 747
New York JFK American 767/74L, British Airways DC10, Virgin 747
Newark Continental 747
Orlando British Airways 747
Philadelphia TWA 767
St Louis TWA 747