Problem solved by using "createUnit" instead of "createVehicle" for the AI gunner.
Here the functional script :
_piloteSu = _this select 0;
_posSu = getmarkerpos "su34";
_sukoi = createVehicle ["Su34",_posSu,[], 0,"FLY"];
_sukoi setPos [(getPos _sukoi select 0),(getPos _sukoi select 1),900];
_piloteSu moveInDriver _sukoi;
_groupPilot = createGroup west;
"USMC_Soldier_Pilot" createUnit [_posSu, _groupPilot,"tireur = this", 0.5, "PRIVATE"];
[tireur] allowGetIn true;
[tireur] orderGetIn true;
tireur assignAsGunner _sukoi;
tireur MoveInGunner _sukoi;
if (true) exitWith {};