Never Compromise Gray Zone GZ4 Belly Putter. The Never Copmromise Gray Zone putter series consists of five different models, all featuring 303 stainless steel construction with aerospace-thin wall technology in the face and complemented by an all-new TransMatter II™ insert located in the back cavity of the putters. TransMatter II gives a more solid feel to help get you in the zone! The GZ4 Belly is a flange-back mallet with a centre shaft fitting.