The 27-year-old appeared to be struggling during a scrappy opening three rounds of the bout.
However, Gavin began to up his workrate during the middle rounds and after a dominant eighth round, he outlasted the tiring Witter to claim an unanimous points decision.
Gavin told Box Nation: "It means everything. A year ago, no-one ever thought I would even win an English title and now I am back where I belong."
Thursday's win extends Gavin's unbeaten professional record to 14 fights.