It wasn’t possible to plan trains to run via Horsham-Dorking because it was closed for an engineering possession (yes, that happened again...). That route reopened at about 1530 when Network Rail and their contractors were fortunately able to lift the possession once they had cleared the work being undertaken. (Obviously that could potentially make that route more vulnerable to suspension in future, suffering as it does with ongoing embankment issues, electrical faults and suchlike.)
The Gatwick Airport - Three Bridges bus was timetabled to run every 5 minutes, with many more buses on standby. Eventually a total of 55ish extra buses were deployed/resourced in the middle of the day - quite an impressive feat for a Bank Holiday Sunday lunchtime. Unsurprisingly, this cleared the queues in an impressive time.
Unfortunately the type of buses being used - and standing passengers - meant the buses were forbidden from the motorway and as such each circuit took longer. The 12 per hour bus frequency also did not quite seem to account for the combination of air travellers and the sheer volume emerging from the trains from London. The Gatwick emergency queuing system, however, is a method which has been planned and practiced for many years, and was fully implemented for one of its first times in anger - hence various references on Twitter to being redirected via long detours on the ramps. It can swallow one hell of a lot of people, so this meant the service could still run, just flowing poorly until extra buses arrived.
Nothing I have seen indicates the problem can realistically be able to be deemed as severe for people returning from the South Coast in the evening, as although everywhere was crowded, additional trains and buses (kept on duty from earlier, or resourced later) did seem to pick up people promptly.