I understand the point about how giving the passengers a choice could be problematic given the need to sort out baggage; surely the only sensible decision would have been to let passengers disembark, collect their luggage and offer to lay on coaches for those who wanted to be taken to Gatwick (probably with a substantial wait for those coaches, but hey, at least pax would have been given the option to make their own alternative arrangements). IMHO, the difference between arriving at Gatwick and Heathrow is neglible and I would imagine only a very few passengers (elderly/infirm) would be reluctant to make their own onward travel arrangements.
Keeping passengers on board for this length of time in these circumstances is completely unacceptable. Also, some people will have been travelling with children; I don't think my two are particularly unruly, but the thought of trying to keep them entertained for 9 hours [I]at the end of a long flight[I] doesn't bear thinking about.