Gay will compete for one of 12 spots on the USA's official roster for the FIBA World Cup in Spain. Durant cited physical and mental exhaustion as his reasons for withdrawing.
Following Paul George's traumatic leg injury suffered during an intrasquad scrimmage and Durant's withdrawal, Team USA was left with Klay Thompson, Chandler Parsons, Kyle Korver and Gordon Hayward as options on the wing.
Last year, after being traded to the Sacramento Kings, the 6-foot-8 Gay averaged 20.1 points, 5.5 rebounds and 3.1 assists per game on 48 percent shooting.
Gay won gold with Team USA at the 2010 World Championship in Turkey.
The forward's addition brings the U.S. roster total to 16, a number that must be trimmed to 12 by Aug. 29, the day before the World Cup begins. The U.S. team has an exhibition game against Brazil scheduled for Aug. 16 at the United Center in Chicago.