A golf ball including a thermoplastic core having an outer diameter of 1.51 inches to 1.59 inches; an outer cover layer; and an inner cover layer between the core and the outer cover layer. The thermoplastic core has been exposed to a gradient-initiating solution comprising a solvent, free radical initiator, and a reactive co-agent comprising a metal salt of an acrylic or methacrylic acid, a mono- or multi-functional acrylate or methacrylate, or having the chemical structure where R=linear, branched, or aromatic C1-C14 and n=1-5, such that the hardness of the outer surface is different than the hardness of the geometric center to define a positive or negative hardness gradient of 5 Shore C or greater.