Agradecimentos: The authors thank the LNLS for providing beamtime to this project, LNNano for assistance in the device construction, and Lab Nanomateriais at UFMG for allowing the use of the atomic-layer transfer system. We thank Y. Petroff, J. Roque, and H. Westfahl for the in-depth discussions and for stimulating the research. Thiago M. Santos, Vinicius O. da Silva, and Neaspec GmbH are acknowledged for the technical assistance in the experiments. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and JSPS KAKENHI grant no. JP15K21722. ARC, I.D.B., and L.C.C. appreciate the financial support from CAPES, Fapemig, CNPq, and INCT/Nanomateriais de Carbono. C.D. acknowledges financial support by CNPq. M.B.R and A.B. acknowledges financial support through the AFOSR grant no. FA9550-17-1-0341. B.T.O. and M.B.R. acknowledge support from the U.S. Department of Energy, Office of Basic Sciences, Division of Material Sciences and Engineering, under award no. DE-SC0008807 Abstract: Light-matter interaction in two-dimensional photonic or phononic materials allows for the confinement and manipulation of free-space radiation at sub-wavelength scales. Most notably, the van der Waals heterostructure composed of graphene (G) and hexagonal boron nitride (hBN) provides for gate-tunable hybrid hyperbolic plasmon phonon-polaritons (HP3). Here, we present the anisotropic flow control and gate-voltage modulation of HP3 modes in G-hBN on an air-Au microstructured substrate. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP3 waves in both hBN Reststrahlen bands and observe directional propagation across in-plane heterointerfaces created at the air-Au junction. The HP3 hybridization is modulated by varying the gate voltage between graphene and Au. This modifies the coupling of continuum graphene plasmons with the discrete hBN hyperbolic phonon polaritons, which is described by an extended Fano model. This work represents the first demonstration of the control of polariton propagation, introducing a theoretical approach to describe the breaking of the reflection and transmission symmetry for HP3 modes. Our findings augment the degree of control of polaritons in G-hBN and related hyperbolic metamaterial nanostructures, bringing new opportunities for on-chip nano-optics communication and computing CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAIS - FAPEMIG Fechado