The standard method for growth rate determination in semiconductor thin films, by Molecular Beam Epitaxy (MBE), is through RHEED intensity oscillations prior to device layer epitaxy. High quality III-Nitride epitaxy occurs with metal-rich surfaces and under step-flow growth conditions, which do not produce RHEED oscillations. This article demonstrates the capability to monitor the growth rate of gallium nitride (GaN), at any point during film growth with high fidelity, under step-flow growth conditions. RHEED intensity vs. time measurements determine the growth rate by Metal Modulated Epitaxy (MME). Utilizing differential analysis, a factor of 2x improvement in accuracy is demonstrated, with a Standard Error less than 4%. Complementary analysis with X-Ray Diffraction and RHEED identify the Ga bilayer thickness as 2.34 ML +/- 0.08 ML, representing the first time RHEED analysis has been used to characterize the thickness of the Ga bilayer on GaN.