The magnetization dynamics of ferrimagnets, (FeCo)1-xGdx, driven by the spin transfer torque is studied using the atomistic model. A nonuniform switching process is observed, i.e., the switched area expands to the entire sample via domain wall motion. The atom distribution significantly affects the formation of the earliest switched region. In addition, during the propagation of the switched region, a magnetization vortex appears when two domain walls meet each other. The motion of the vortex leads to the back-and-forth switching of magnetization. Our study helps to understand the switching in ferrimagnets.