The event was won by Britain's Ruth Edge, whose decision to ride long-time mount, the 15-year-old Two Thyme, paid off handsomely.
"I only had one ride in me today," she explained, "and I chose Two Thyme because he doesn't pull."
Edge is the first rider to retain the championship since Mary King in 1991 but she now joins an elite club of four, Mark Todd, Mary King and William Fox Pitt, who have won three times, while Two Thyme is also one of just three horses that have won the title twice. King William and Moon Man are the other two.
Edge, 29, who headed the largest HSBC FEI World Cup Eventing field of 2009, brought the day to a thrilling climax when finishing 7.3 penalties clear of Oliver Townend (GB), winner of the Mitsubishi Motors Badminton Horse Trials and current leader in the HSBC Classics, who in turn was 7.2 penalties ahead of Bill Levett (Aus) on Hippolyte.
"Two Thyme hasn't run much in the last two years - this is only his third time this year - so for him to go like that and look after me so well is just incredible," Edge said.
"He's a deceptively fast horse and didn't feel tired at the end, so I was pleased. Gatcombe is the sort of course where you have to go as fast as you can to win and plan where you are going to give your horse breathers."
Townend, who finished second, said he thought he was "a goner at the gate [fence 9] but we survived. I was thrilled with the Flint Curtis; it's great to get a run like that under your belt before the Europeans."
Levett said third was Hippolyte's best result to date: "right up there with finishing third at Saumur. He's been a consistent horse."
Powell and Lenamore were slightly over time on the cross-counry - as were most starters - adding 9.2 penalties to their dressage score of 46.7.
Lucinda Fredericks (Aus) and Mary King (GB) jointly held the lead after the first two phases, but Fredericks withdrew her Rolex Kentucky Three Day Event winner Headley Britannia because the mare has been undergoing embryo transfer.
King then suffered a frustrating mishap with Imperial Cavalier when missing the distance at the Land Rover at the Folly (fence 16), a line of related, downhill tyre fences, and she fell off over the horse's head when he jinked to the right.
However, King, who is lying 2nd in the World Cup rankings, is planning to take her mare Kings Temptress, on whom she finished 7th at Gatcombe, to the Final at Strzegom in Poland in a fortnight's time.
Clayton Fredericks (Aus), lying fourth, also withdrew Ben Along Time, as he is saving him to defend his title in Poland.
The cross-country at Gatcombe is renowned among riders as being one of most testing, because it runs up and down the side of a steep valley beneath the Princess Royal's house, providing a thrilling spectacle for the crowds and a test of nerve and stamina for riders and horses.
William Fox-Pitt (GB) also fell foul of fence 16, with a run-out on Macchiato, and Rodney Powell (Zin Zan ll, GB) and Mark Todd (Gandalf, NZ) both had run-outs at the narrow fence at the top of the steps at fence 8, and Stuart Fitzgerald (South Africa) had a fall when Classic Diamond Charm slipped backwards here.
The HSBC Corners at 13 also proved influential - Ireland's Geoff Curran was among the riders to run out there - and many competitors opted for the slow route.
The optimum time is always a challenge at Gatcombe, and Nicola Wilson (GB) who will ride as an individual for Great Britain at the HSBC FEI European Eventing Championship in September on her brilliant horse Opposition Buzz, was the only rider who achieved this. Benjamin Massie on Haston d'Elpegere, 15th, one of a strong contingent of French riders at Gatcombe, was second fastest with 2 penalties.
85 started the dressage
9 nations represented: Australia, Belgium, Brazil, France, Germany, Great Britain, Ireland, New Zealand, South Africa
Ground Jury was Eric Smiley (IRL, president), Sandy Phillips (GBR), Angela Tucker (GBR) Marks range from 36.0 to 78.3
82 started the Jumping
1 rider eliminated in the Jumping, Andrew Nicholson (NZL) on Armada for taking the wrong course
There were 24 clear rounds
70 Started the Cross-Country
There were 40 clear rounds
Only 1 rider, Nicola Wilson (GBR), achieved the optimum time
Cross-country course-designer was Capt Mark Phillips (GBR)
Technical Delegate was Jonathan Clissold (GBR)
The winner, Ruth Edge (GBR) was taking her 3rd national (British) title, having won in 2004 (on Two Thyme) and 2008 (on Mayhem).
1 Ruth Edge/Two Thyme (GBR) 36.9 + 0 + 3.2 = 40.1
2 Oliver Townend/Flint Curtis (GBR) 40.6 + 4 + 2.8 = 47.4
3 Bill Levett/Hippolyte (AUS) 45 + 4 + 5.6 = 54.6
4 Polly Stockton/Westwood Poser (GBR) 46.9 + 0 + 8.8 = 55.7
5 Caroline Powell/Lenamore (NZL) 46.7 + 0 + 9.2 = 55.9
6 Nicola Wilson/Opposition Buzz (GBR) 53.9 + 4 + 0 = 57.9
7 Mary King/Kings Temptress (GBR) 47.1 + 4 + 9.2 = 60.3
8 Maxime Livio/Jaipur ll (FRA) 46.4 + 0 + 14.8 = 61.2
9 Andrew Nicholson/Nereo (NZL) 55.8 + 0 + 8.4 = 64.2
10 Nick Gauntlett/Chilli Morning (GBR) 50.2 + 0 + 14 = 64.2
Standings (after 8 of 9 qualifiers)
1 Megan Jones (AUS) 203
2 Mary King (GBR) 165
3 Alexandr Markov (RUS) 125
4 Simone Deitermann (GER) 117
5 Andreas Dibowski (GER) 104
6 Kai Ruder (GER) 92
7 Ilya Anisin (BLR) 74
8= Marina Kohncke (GER) 71
8= Patricia Ryan (IRL) 71
8= Kelly Prather (USA) 71
8= Ruth Edge (GBR) 71
12= Buck Davidson (USA) 70
13= Heelan Tompkins (NZL) 67
13= Georgia Clifford (AUS) 67
13= Julia Mestern (GER) 67
13= Peter Thomsen (GER) 67
13= Michael Ryan (IRL) 67
13= Aistis Vitkauskas (LTU) 67
13 = Jennifer Wooten-Dafoe (USA) 67
13 = Oliver Townend (GBR) 67