a variation of the name Geshem. (Nehemiah 6:6) (B.C. 446.)
International Standard Bible Encyclopedia
gash'-mu, gash'-moo (gashmu):
A form of the name GESHEM (which see), found in Ne 6:6 (compare Ne 6:1), "And Gashmu saith it." According to BDB the same termination -u is found in Nabatean proper names.