Gavin, Britain's only ever world amateur champion, will face tough Welshman Jason Cook in an eliminator for the British light-welterweight title, replacing Bellew's Commonwealth light-heavyweight title defence against Ovill McKenzie.
The Birmingham star was already featuring on the undercard in an eight-rounder, but is now elated that he's the main event on the show.
He was due to box last December in Liverpool against Dean Harrison, but had to withdraw because of flu.
Cook formerly held the European lightweight title and last year battled out a draw with British light-welterweight champion Lenny Daws.
"I'm fit, ready to go and can't wait for this Saturday knowing that a win will get me closer to a shot at the current British champion Lenny Daws," said Gavin.
"I really feel for Tony and am gutted that he's had to pull out, he really wanted to get in the ring again with McKenzie and do a job on him after their fight last December.
"Tony will at least be happy that his best mate is taking his top spot on the show though!
"This is great opportunity for me to really put myself in the limelight and show what I can do against a tough former European champion.
"Cook has got plenty of experience and he will come to give me a hard fight, I'm ready for the step up to face him and show that I'm in the new kid on the block.
"I've stated in the past that Daws is the man I want and that British title that he's holding for me, a good win over Cook will put me close to him.
"This will only be ninth fight but. like James DeGale, I want that Lonsdale belt around my waist."