In this paper, we present a comprehensive theoretical study of the electronic states in gapped graphene in the presence of a charge impurity and a magnetic field. We focus on the impurity-induced modification of the Landau levels under the influence of an attractive or repulsive Coulomb potential. Since analytical solutions of the Dirac equation are not available in this case, the study is carried out using the Wentzel-Kramers-Brillouin (WKB) approach. Special attention is paid to the screening of the Coulomb potential in the presence of metallic gates situated at some distances from graphene sheet, as is commonly the case in experiments. Using both analytical and numerical methods, we determine the energy spectrum as a function of the magnetic field strength and impurity charge for screened and unscreened Coulomb potentials. Our results reveal a significant dependence of the energy levels for sample-to-gate distances of less than 50 nm.