The ionospheric horizontal gradient is the variation of electron density with latitude and longitude which can cause the azimuthal deviation to the Global Positioning System (GPS) ray path. The effect of an ionospheric horizontal gradient is more pronounced in the low-latitude region (equatorial) and mid-auroral (trough) or polar regions than at midlatitudes. However, most of the time, the presence of the ionospheric horizontal gradient will be ignored over the equatorial region. In applications that use the Differential Global Positioining System (DGPS) method, this neglect can cause a significant amount of error in the final user positioning. In this work, a 3D electron density model was developed for a desired region of latitude and longitude at 350 km by using the value of electron density from the International Reference Ionosphere (IRI). This model represents the actual formation of the ionosphere in 3D (inhomogeneous). This model then will be incorporated as the new electron density subroutine in the Jones 3D Ray Tracing program in order to determine the ionospheric horizontal gradient effect for GPS signals that passes through the equatorial ionosphere.