The realization of next-generation medium to high power vertical GaN devices relies heavily on the epitaxial growth of GaN on commercially available, high-quality GaN substrates. Recent advances in commercially available GaN substrates have enabled the design and fabrication of vertical GaN devices that demonstrate promise and feasibility, but are limited by materials challenges of homoepitaxial GaN growth. Although homoepitaxial GaN is expected to provide more control over growth characteristics (e.g. crystallinity and doping), its reliability and reproducibility suffer at the hand of the native GaN substrates available to date. The variations in commercial GaN substrates include defect density, surface roughness, wafer bow, and photoluminescence properties. Thus, elucidating the role of GaN substrate properties on the growth and characteristics of resulting homoepitaxial GaN films has emerged as a new challenge towards next-generation GaN power devices. In this presentation, a systematic study of the morphological, electrical, and optical properties of GaN films on GaN substrates will be presented to identify substrate and epitaxial film metrics that administer high mobility, low leakage current, low on-resistance, and high breakdown voltage.