Scelzi grew up in Fresno, California, in a motorsport family. His father Gary Scelzi is a former drag racing champion, and his older brother Dominic Scelzi is also a sprint car driver. Gio began racing in the micro-sprint ranks at Plaza Park Raceway, and at just nine years old he posted 24 wins in a single season at Lemoore Speedway. He attended Clovis Online High School through a blended learning model combining in-person and online instruction.
At age sixteen, Scelzi relocated from Fresno to Indianapolis, Indiana, to be closer to the heart of sprint car competition. That move quickly paid dividends: in September 2017, he became the youngest winner in World of Outlaws history by winning a preliminary event at the National Open at Williams Grove Speedway in Pennsylvania. The milestone established him as one of the most precocious talents in American dirt-track racing.
In 2019, Scelzi became the youngest driver to win a feature at Knoxville Raceway, threading the high groove around the track to seize the lead. That same year he claimed his first United States Auto Club midget victory, winning at Placerville Speedway. The double-discipline success underlined his versatility on dirt.
Scelzi opened 2020 at the Chili Bowl Nationals in Tulsa, Oklahoma, before announcing a limited sprint car schedule in the Midwest in partnership with Guy Forbrook. Simultaneously, he committed to a full ARCA Menards Series West campaign for Bill McAnally Racing in the No. 16 entry — a planned detour into stock-car-style racing on paved ovals.
He took his first Pro Late Model win at New Smyrna Speedway in February 2020, benefiting from a last-lap incident that removed the top two contenders. His ARCA West season featured a career-best second-place finish at the Las Vegas Motor Speedway Bullring; a race later, he used a bump-and-run on Taylor Gray in the penultimate lap to claim his first ARCA West victory, finishing fifth in the season championship standings.
During the summer of 2020 he returned to dirt, rejoining his family sprint car team for All Star Circuit of Champions events and competing with Tucker Boat Motorsports during Indiana Midget Week.
In 2023, Scelzi joined the World of Outlaws Sprint Car Series full time, targeting Rookie of the Year honors. His tenure with KCP Racing lasted through the Double Down Duels at Eldora Speedway in July 2025. Following Tyler Courtney's season-ending injury, Scelzi joined Clauson-Marshall Racing to complete the High Limit Racing season, continuing his association with Spire Motorsports in the series.
Scelzi is the son of Gary Scelzi and the younger brother of sprint car driver Dominic Scelzi. He currently resides in Grimes, Iowa.