The prospect of an end to the Iran-Iraq war is raising the possibility that the United States will be able to reduce its military presence in the Persian Gulf, administration officials say. White House spokesman Marlin Fitzwater alluded to that possibility Monday after Iran agreed to accept a United Nations resolution calling for a cease-fire in its war with Iraq. "We have always said that as hostilities end and as the war ends, we were willing to consider a withdrawal," Fitzwater told reporters in Santa Barbara, Calif., where President Reagan is on a weeklong vacation.