The gyromagnetic nonlinear transmission line (GNLTL) is a special transmission line loaded with ferrite cores, which can be used as a kind of wideband high-power microwave (HPM) source due to the pulse modulation mechanism. This article carried out a series of simulations and analyses focused on the RF characteristics of GNLTL. Based on the classic transmission line model (TLM) method, a numerical model linking the transmission line equations and the microscopic magnetization dynamics equations was established. The simulations systematically studied the effects of key parameters on the output characteristics especially the RF characteristics, including the effects of transmission line geometry, excitation pulse, magnetic material, and the biasing magnetic field. The simulation results and analyses revealed that almost each parameter has effects on the output, and the effects of different parameters are different. The research introduced in this article preliminarily reveals how the output microwave varies with the parameters and provides a method to find the appropriate parameter values of a GNLTL.