We present a general simulation framework for assessing the performance of nanoscale devices that combines several powerful and widely used open-source codes, and based on minimal but chemically accurate tight-binding Hamiltonians obtained from density-functional theory calculations and using maximally localized Wannier functions to represent the electronic state. Transport properties are then computed within the nonequilibrium Green's function formalism. We illustrate the capabilities of this framework applying it to a transistor with generic gate geometries, i.e., a double-gate nanoscale field-effect transistor where the channel is formed by graphene nanoribbons terminated with hydrogen, fluorine, and OH groups.