Gemma won bronze in the women's club throw at the Olympic Stadium, in a new European record.
The 28-year-old was competing in the F31/32/51 class at the Games, and her best effort of 20.50m was comfortably enough to give her third spot.
She scored 1015 points for the throw, which was almost seven metres better than fourth-placed Catherine O'Neill of Ireland.
Morocco's Maroua Ibrahmi won gold with a world record throw of 23.43m, while Algeria's Mounia Gasmi was second with 22.51m.
Even Gemma's worst throw out of her six attempts - her mark of 17.51m in the opening round - would have been enough to win her the bronze.
It was a triumph for Gemma, coming four years after she made her Paralympics debut in Beijing.
Her best performance in 2008 was a seventh place in the shot putt.
Gemma, who comes from Truro and now lives in Warrington, won a shot putt bronze at the 2010 Commonwealth Games in Delhi.
She won bronze at last year's IPC Athletics World Championships in New Zealand in the club throw.