The time evolution of a Gamma-ray burst (GRB) is associated with the evolution of a supernova remnant (SNR). Two laws of motion in the classic and relativistic regime are analyzed. A simple law for the 3D diffusion is reviewed. The time evolution of the flux of a GRB is modeled introducing a classical radius-time relationship for the advancing SNR. The adopted radiative model for the GRB in the various e.m. frequencies is the synchrotron emission. The X-ray ring which appears a few hours after a GRB is simulated adopting two models.