Gavin, 27, claimed a unanimous points decision over veteran Witter to extend his unbeaten professional record to 14 fights.
"It means everything," Gavin told Box Nation. "A year ago, no-one ever thought I would even win an English title and now I am back where I belong."
The Birmingham-born boxer began in cautious mood before growing into the bout and landing the best punch of the contest in the eighth round.
Witter, 38, invited Gavin in by dropping his workrate and hands as the fight progressed but was docked a point for holding in the 10th.
Witter, who again employed his switch-hitting style, raised his game towards the end, but it was too little too late and Gavin landed the belt after the judges scored the fight 119-109, 117-110 and 117-112.