In this work, we analyze in detail the modal properties of a graphene-based planar waveguide (GPW) in the terahertz (THz) frequency range. The structure consists of a graphene sheet placed on the top of a grounded dielectric slab. As is known, the surface conductivity of the graphene sheet can easily be tuned with a bias voltage via electric-field effect; we show here how such a bias affects the propagation features of both TM and TE modes supported by the GPW. An extensive dispersion analysis is performed for complex modes in both guided and radiative (leaky) regimes, considering also dielectric and metal losses as well as nonlocal effects in graphene. In particular, we focus on the behavior of the fundamental leaky modes since they exhibit quite interesting radiation features for suitable values of the bias. These results are very promising for the development of reconfigurable leaky-wave Fabry-Perot …