Preheat a gas grill over medium-high heat, to about 425 degrees.
In a small mixing bowl stir together dijon mustard, mayonnaise, garlic and season with salt and pepper to taste.
Pound thicker portions of chicken with with a meat mallet to help even the thickness. Brush top side of chicken pieces with plenty of the mustard sauce.
Place on preheated grill, sauce coated side down, then brush the uncoated side with mustard sauce.
Cover and grill until chicken registers 165 degrees in center of thickest portion, about 4 minutes per side.