The decision to add Gay, announced on Friday in a statement by USA Basketball, comes a day after NBA most valuable player Kevin Durant withdrew from the team citing mental and physical exhaustion.
Gay averaged 20 points, 6 rebounds and 2.9 assists in 73 games during the 2013-14 NBA season which he spent with Toronto and Sacramento.
He was also on the U.S. team that captured the gold medal at the 2010 World Championships.
The United States opens play in the World Cup on Aug. 30 against Finland.
(Reporting by Frank Pingue in Toronto)