In essence, a gumbat is a large piece of stick (a bat) covered in gum (preferably post-chewed, bubbalicious works best).
A gumbat is the most efficient of weapons because whilst one is beating an invidiual with the said bat, the gum will effectively rip off all of their body hair.
Suz: take a fucking gumbat to her nappy-headed skull.
or, in more colloquial language
Bob: That math test was so hard, I felt like someone was beating me with a gumbat throughout the whole thing.