This paper studies the gain margins (GM) of multivariable model reference adaptive control (MRAC) systems: the parameter range of a control gain matrix in a designed MRAC system for maintaining the desired closed-loop signal boundedness and asymptotic output tracking. Analytical GM results are obtained for both continuous-time and discrete-time direct MRAC schemes applied to multi-input multi-output (MIMO) LTI systems. The GM problem is also studied for a class of indirect multivariable MRAC systems.