The Market Overton 17-year-old took on her more senior rivals at the Jardy International 3 day event, staged near Paris.
Gaby was riding in the CCI* (one star) three day event section at the event, which was held at one of the largest equestrian centres in France, once home to a world famous racehorse breeding operation.
She was riding the big 17.2hh brown horse Northcroft Frontier, owned by her mother Charlotte, who she started eventing in the spring of 2007.
"Northcroft Frontier was a bit naughty in the opening dressage phase – he bucked in the test," Gaby said.
"It was very, very hot and he wasn't amused at having to work. Despite this it was still a good test and we were lying fourth at that stage."
The horse moved into second after an excellent clear inside the optimum time cross country, and was just one mark off the lead.
Gaby said: "The cross country was quite hard work and very different to the events in Britain.
"The track was very twisty and was on a gallop type surface, not grass. They had rotavated it but the time was hard to get, particularly for a big horse, so I was very pleased with Northcroft Frontier.
"It was also a bit more overcast on cross country day, which helped, although it was still pretty humid."
The scores were very tight at the top coming into the show jumping phase, on the last day. Gaby kept her cool, as the temperatures again soared, and jumped another great clear. When the leader, fellow Brit Dani Evans, had one down, the title came home to Rutland.
"I ended up one mark ahead of French senior international Jean Renaud Adde – I wasn't really expecting to win and certainly not to beat a French rider on home soil," Gaby added. "It was our first time abroad but my horse travelled okay and it was a great experience all round."
Gaby, who trains with Chris White, now has the junior european final trial in two weeks.
She added: "That will be crunch time for the British Junior European Championship team, which will be announced after it, so my horse has just one more event to go well in if he's to make it."