This paper presents the performance gain margins of the recently developed two-stage LQ reliable control, which is a discrete-time version of Veillette's reliable LQ regulator, such that the performance cost is guaranteed to be within a certain bound. It is shown that the desired gain margins satisfy a unified gain margin constraint and a solution to solve this constraint is obtained in terms of linear matrix inequalities. An application of the proposed unified gain margin constraint to design a reliable guaranteed cost controller is illustrated. A numerical example is also given to show the effectiveness of the proposed results.