Nothing GAR about her at all.
I do think some females can be considered GAR, mainly females who are already masculine, like Oscar, Subaru, Youko, Revy and the Major.
Rena isn't GAR. She can be badass yes. Psycho yes. A bit of awesome too in some opinions. But not GAR.
GAR is about manliness. A cute and sweet girl who occasionally becomes psycho is not manly.
This is how I personally define a GAR female: It goes off of an old defination of GAR about a man being so manly even the straightest anon would become gay for him. In my mind a GAR female is a strong formidable woman that you would never try to sleep with, no matter how hot she is, due to the fact she is so damn intimidating. But part of you wishes she was a man. If that was the case, you would drop you pants and grab your ankles. A woman so manly, you wish she would turn into a man, and have their way with you.