The gap breathers (GBs) in a one-dimensional Klein-Gordon diatomic chain are studied under rotating-wave approximation (RWA). The analytical investigation of heavy-particle centered symmetric mode (HS mode) has been implemented under the boundary condition of standing waves. We examine the effects of the coupling constant, nonlinear constant and the mass ratio on the GBs. The GB profiles are sensitive to the coupling constant, nonlinear constant and the mass ratio. As the coupling constant increases, the GB profiles become broader. But, with the nonlinearity and the mass ratio increasing, the GB profiles appear more localized.