  Twisting bilayers of two-dimensional topological insulators has the potential
to create unique quantum states of matter. Here, we successfully synthesized a
twisted bilayer of germanene on Ge2Pt(101) with a 21.8$^o$ degrees twist angle,
corresponding to a commensurate ($\sqrt{7} \times \sqrt{7}$) structure. Using
scanning tunneling microscopy and spectroscopy, we unraveled the structural and
electronic properties of this configuration, revealing a moir\'e-modulated band
gap and a well-defined edge state. This band gap opens at AB/BA stacked sites
and closes at AA stacked sites, a phenomenon attributed to the electric field
induced by the scanning tunneling microscopy tip. Our study further revealed
two van Hove singularities at -0.8 eV and +1.04 eV, resulting in a Fermi
velocity of $(8 \pm 1) \times 10^5$ m/s. Our tight-binding results uncover a
unique quantum state, where the topological properties could be regulated
through an electric field, potentially triggering two topological phase
transitions.
