Quigg will meet the South African after original opponent Nehomar Cermeno was forced to withdraw because of visa issues, and Gallagher has acknowledged that Saturday's bout at the Phones4U Arena is a "tricky" fight.
The 45-year-old told Sky Sports News: "Tricky is an understatement - he's very good. "He's been over here before, stopped Lee Haskins and beat Martin Power twice - both very capable fighters.
"He's very tall and not scared to fight, he's got quick reflexes and all the moves, it's a very dangerous fight and at a weeks' notice too.
"Scott has been in training for nine weeks for a specific opponent with certain habits and to re-adjust that for a really tall kid is a near-impossible task. But he's on fire in the gym and I expect nothing less than a knockout."
Quigg is undefeated in 29 bouts, while Munyai has only lost twice in 27 encounters.