The relentless push toward 6G communications and advanced radar systems demands unprecedented power and frequency metrics from radio-frequency (RF) front-ends. While gallium nitride (GaN) remains the undisputed cornerstone of high-power RF electronics, traditional lateral architectures are colliding with fundamental physical scaling limits. This review systematically maps the evolutionary trajectory of GaN RF devices, spotlighting the critical paradigm shift from lateral surface conduction to vertical bulk and quasi-ballistic transport mechanisms. We first critically assess the physical bottlenecks of lateral high-electron-mobility transistors (HEMTs)—specifically the severe short-channel and localized thermal hotspot effects near the 20-nm gate limit. To mitigate these, we evaluate recent structural interventions, including polarization-driven barrier engineering and 3D FinFET topologies. To transcend planar constraints, we detail the transition toward vertical architectures designed to fundamentally decouple breakdown voltage from the device footprint. For heterojunction bipolar transistors (HBTs), recent breakthroughs in non-destructive epitaxial growth and tunnel-junction contacts are highlighted for conquering long-standing p-GaN ohmic challenges. Furthermore, we explore the frontier of hot-electron transistors (HETs), where nanometer-scale bases enable quasi-ballistic transport to unlock the terahertz (THz) regime. Ultimately, the future of GaN RF electronics hinges not on further geometric miniaturization, but on vertical topological innovations and system-level heterogeneous integration, forging the indispensable hardware foundation for next-generation extreme-power applications.