This paper introduces a virtual gravity controller for underactuated biped robots. A bio-inspired model of passive bipedal walking is used as the basis for the controller's design. An analytical expression of the controller is obtained, allowing on-line implementations of the developed control scheme. Following a design modification tailored to the controller, the robot is able to reproduce its passive gait even on level-ground. The results are verified via independent high-fidelity physics simulations of the real robot's digital twin. The active robot demonstrates significant dynamic convergence to the passive model's dynamics, with only minor motorization efforts. The developed control scheme showcases robustness and energetic efficiency, and leads the way to a design-oriented approach in active biped locomotion.