Gramigni was born in Florence, Italy. He developed through the amateur ranks before making his Grand Prix debut in 1990 in the 125cc class on an Aprilia, ending his maiden season in ninth position in the championship standings.
Gramigni won his first Grand Prix at the 1991 Czechoslovak Grand Prix and closed that season ranked seventh with 90 points. His victory in Czechoslovakia was also notable as the first Grand Prix win in world championship competition for the Aprilia factory.
The 1992 season brought Gramigni his greatest success, though it came through remarkable adversity. He had started the year impressively, winning at Shah Alam in Malaysia among the opening rounds. Before the Spanish Grand Prix, however, he was struck by a car while riding an enduro motorcycle, sustaining fractures of the tibia and fibula with ligament involvement in his foot. Under the care of Doctor Claudio Costa and the Mobile Clinic — the travelling medical unit that serves the Grand Prix paddock — Gramigni returned to racing just 19 days after the accident at the Italian Grand Prix at Mugello. He raced with a cast on his foot covered in leather to resemble a boot, finishing eleventh.
One week later, still in the cast, he took fourth at the European Grand Prix. His consistency through the second half of the season, including a victory at the Hungarian Grand Prix, proved decisive. While rivals aboard Honda machinery had erratic campaigns, Gramigni remained steady. He claimed the 125cc World Championship by 16 points over second-placed Italian Fausto Gresini, simultaneously handing Aprilia its first world championship title in the company's history.
Gramigni moved up to the 250cc class for 1993, riding first for the Gilera team before returning to Aprilia. The season was difficult, yielding only two championship points. He remained in the 250cc class for 1994 with Aprilia and 1995 with Honda before a two-year break from Grand Prix racing. He made a single appearance in the 500cc class at the 1997 Malaysian Grand Prix before closing his Grand Prix career.
From 1998 onwards, Gramigni competed in the Superbike World Championship across several seasons between 1998 and 2005. He achieved his most significant domestic result in 2004, winning the Italian Superbike Championship with Yamaha Team 391 Racing.
Gramigni's 1992 world title stands as a landmark moment for Aprilia — an Italian manufacturer that would go on to become one of the most successful factories in the small-displacement classes of Grand Prix racing. His comeback story, racing with a fractured leg within three weeks of injury and continuing to mount a successful championship challenge, remains one of the more remarkable episodes in 125cc history.