The gapper works. Southern Raider is correct in that when training on roper weaons handling, charging, mag changes etc. you will hold most of the weight of the weapon in your firing hand, keeping the muzzle on target and charging the weapon and changing magazines with your weak hand. Depending on how you hold the weapon, you will soon rub your middle finger raw from either the corner of the pistol grip or the gap between the trigger guard and the pistol grip. The gapper fixes that. I started shooting with nomex flyers gloves to prevent that, but it got very hot training in July and August. Jim Crews sold me on the Gapper. He gave me one to try during a break in class and sold me on the concept.
A field expediant is to put t tightly rolled foam earplug in the gap.
A word of caution for military and police shooters. If you change the grip or controls on your personal weapon and then have to fight with a different one for whatever reason, you may not be as efficient. I'd keep everything the way all the ones my agency issued are.
Jeff