Unless the Government mandates for no use of private email, it is perfectly in order. In the case of any institution, email confirmation has to be sent by Speed post as hard copy. When there is no bar on such using of g mail, the order is legal particularly when the recipient accepted such mails in the past and the department has been using such mail for daily correspondence, there can be no illegality. The orders can even be oral and the orders are not for employees alone, and the concerned authority has to take charge and relieve him. When the employee is relieved, he can not question the authority of relieving officer.