From The Collaborative International Dictionary of English v.0.48 :
gumshield \gumshield\ n. (Sport)
A piece of athletic equipment that protects an athlete's
mouth.
Syn: mouthpiece.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) :
gumshield
n 1: (especially boxing) equipment that protects an athlete's
mouth [syn: mouthpiece, gumshield]