The Court of Arbitration for Sport says its special Olympics court rejected his urgent appeal against exclusion by the French team for doping.
Gezzar had been entered in the 3,000 metres steeplechase heats which start Friday.
The French athletics federation provisionally suspended Gezzar after he tested positive for the blood boosting hormone EPO at the national championships in June.
The IAAF world athletics body then extended the ban globally.
CAS says Gezzar did not show that errors were made "during the anti-doping control procedure."
The 32-year-old Gezzar served a two-year suspension for doping in 2006. He faces a life ban for a second offence.