Gameiro joins fellow new signings Ivan Franjic and Steve Kuzmanovski in Melbourne as he looks to bounce back from a serious knee injury which ruled him out for the majority of last season.
The 22-year-old said he was eager to continue his rehabilitation at the City Football Academy in Melbourne.
"I'm ahead of schedule at the moment, so I am doing well," Gameiro said. "The facilities here are excellent and I am very happy to be here at the club and continuing my rehabilitation in the Academy's fantastic gym.
"I believe the club is heading in the right direction, have the foundations right and have the potential now to achieve success and I want to be a part of it."
After a spell with Fulham in the UK - during which time he had loan stints with Hayes & Yeading, FC Eindhoven and Wellington Phoenix - Gameiro spent two seasons with Sydney but will now join John van't Schip's Melbourne City.
"We have been monitoring Corey's progress and believe he has huge potential with us at Melbourne City FC," Van't Schip said.
"Corey has proven he has what it takes to be a successful striker in the A-League, showing that he has the desire, skill and hunger to succeed.
"He is a great professional and is excited about his move to Melbourne and we are looking forward to welcoming him to the club."
