This study reveals the unified geometric origin of the gravitational constant $G$ and the speed of light $c$. By introducing the rotational scaling length $l_\Omega$, defining the helical divergence strength $Q = 4\pi c l_\Omega^2$ and the space–mass coupling relation $M = \mu_g l_\Omega$, we establish a one-to-one correspondence between mass and geometric scale. Under the unique bridging law $GM = \frac{c}{l_\Omega} Q$, we rigorously derive the invariant $\frac{GM}{4\pi c^2 l_\Omega} = 1 \Rightarrow G = \frac{4\pi c^2}{\mu_g}$.

This result demonstrates that G and c are not independent constants but are jointly fixed by the space–mass coupling constant $\mu_g$ and geometric structure. Using observational data from solar redshift, light deflection, planetary perihelion precession, and both Jupiter’s satellites and the white dwarf Sirius B, we verify the universality and cross-modal consistency of this relation. Geometric scaling not only explains the origin of G but also establishes a unified theoretical framework for future high-precision astronomical observations and constant metrology. This offers a more geometric expression than general relativity and proposes testable metrological predictions.