I've never had to wait on the tarmac at Gatwick for a stand. I've never heard of anyone waiting for an hour, not to say that it's not possible but it's not something I've heard of.
Immigration is quick and easy if you're a US citizen as you can use the e-gates (unless you're accompanied by anyone under 12), the time it takes to get through these is seconds however your wait time is dictated by the queues waiting to use them. Sometimes I've landed and walked straight through without queueing whereas other times I've had a wait of no more than five minutes. The biggest factor affecting timings is the number of flights that land around the same time and where the passengers are disembarking (will you all meet at the arrivals hall at the same time or will some passengers have a longer transit time than you).
You will have to go through security prior to departure if collecting luggage. This typically is no longer than 20 minutes.
There is no passport control departing the UK other than the brief passport and boarding pass check at the boarding gate.
Would I be comfortable with five hours? Personally no but that's because I'm highly risk averse and my PTSD would go into overdrive but other people would be content I'm sure.