This paper presents a compact virtual source (VS) model to describe carrier transport valid in both unipolar and ambipolar transport regimes in quasi-ballistic graphene fieldeffect transistors (GFETs). The model formulation allows for an easy extension to bi-layer graphene transistors, where a bandgap can be opened. The model also includes descriptions of intrinsic terminal charges/capacitances obtained selfconsistently with the transport formulation. The charge model extends from drift-diffusive transport regime to ballistic transport regime, where gradual-channel approximation (GCA) fails. The model is calibrated exhaustively against DC and S-parameter measurements of GFETs. To demonstrate the model capability for circuit-level simulations, the Verilog-A implementation of the model is used to simulate the dynamic response of frequency doubling circuits with GFETs operating in the ambipolar regime.