Primary aliphatic amines yield, as seen with anilines, diazonium salts on treatment with HNO2. Aromatic diazonium salts are stable, but that is not the case with aliphatic diazonium salts, which break down fast to yield a mixture of alcohols and alkenes.
RNH2 + HNO2 __> [RN2+](unstable) __H2O__> N2 + ROH
In most cases a complex mixture of alcohols and alkenes are obtained.
I doubt that KMnO4 oxidation of GABA would yield anything useful, unless you have a reference to prove me wrong.
BTW NaNO2 is not *highly* toxic, not like NaN3 or cyanides or alikes. It is toxic yes, but you should always wear protection anyway, and no-one is telling you to eat it.