The Australia U20 international was released by Fulham recently but has swiftly found a new club with the Sky Blues pouncing for his signature.
Gameiro, who spent time on loan with Wellington Phoenix last season, was looking forward to playing alongside star striker Alessandro Del Piero under former Socceroos boss Frank Farina after his duties at the U20 World Cup in Turkey.
"I'm really excited to be joining Sydney FC," Gameiro said.
"I grew up in Wollongong and it will be like coming home. I'm looking forward to joining my new team mates in preseason training, playing alongside a great of the game like Alessandro Del Piero and learning from Frank Farina and the Sydney FC coaching staff.
"Although, I hope to help the U20s progress further at the World Cup here in Turkey."