Catalyzes the oxidative demethylation of 4-methylaminobutanoate produced from the pyrrolidine ring of nicotine. To a much lesser extent, can also use sarcosine as substrate, but is not active against dimethylglycine, methylaminopropionitrile, methylaminopropylamine, and alpha-methylaminobutanoate. 4-(methylamino)butanoate + H2O + O2 = 4-aminobutanoate + formaldehyde + H2O2 Binds 1 FAD covalently per subunit. kcat is 800 sec(-1) with 4-methylaminobutanoate as substrate. kcat is 4 sec(-1) with sarcosine as substrate. Alkaloid degradation; nicotine degradation. Is transcribed only in the presence of nicotine under the control of the transcriptional activator PmfR. Forms part of an operon with purU, folD, nepA and nepB. Belongs to the GcvT family.