The steeplechaser tested positive for erythropoietin (EPO) at the French championships in June and was withdrawn by the French Athletics Federation.
The ad hoc division of CAS said it had rejected a challenge to Gezzar's temporary suspension after a hearing in which the athlete's agent testified by telephone.
"The athlete did not provide sufficient elements to persuade the tribunal to lift the provisional suspension," CAS said in a statement.
A request to CAS, involving French modern pentathlete Jean-Maxence Berrou, would be heard on Friday according to an earlier statement.
Berrou, with the backing of the French governing body for the sport, has asked that he be allowed to compete at the Games in place of Polish competitor Lukasz Kletot who has withdrawn from the event.
An initial decision by the International Modern Pentathlon Union replaced Kletot with Irish athlete Arthur Lanigan-O'Keefe.