No - the stability margin is positive.
For low frequencies, the phase is -180deg - that means: The minus sign at the summing node is included in your analyses. So you have the correct loop gain function - and the stability limit would be reached when the phase is -360 deg (or 0 deg) for positive magnitudes.
No problem at all - you are "deep" in the stability region.
(Note: Only when the minus sign at the summing node is NOT included in the loop gain function - as shown in some books/articles - the stability limit is at -180deg).
Comment: Because you have mentioned the term "gain margin" I have assumed that the shown function (and the plots) is the loop gain of a circuit with feedback - and you are interested to know if the closed-loop will lead to a stable system. Is this correct? If not, the question regarding gain margin makes no sense.