We propose the theory of transport in a gate-tunable graphene p-n junction, in which the gradient of the carrier density is controlled by the gate voltage. Depending on this gradient and on the density of charged impurities, the junction resistance is dominated by either diffusive or ballistic contribution. We find the conditions for observing ballistic transport and show that in existing devices they are satisfied only marginally. We also simulate numerically the trajectories of charge carriers and illustrate challenges in realizing more delicate ballistic effects, such as Veselago lensing.