Interest in the mmWave and sub-THz bands for next-generation communication and radar systems is increasingly growing due to the available electromagnetic spectrum. This has led to an urgent need to develop low-loss and low-fabrication-cost technologies that perform well at these frequencies. The Gap Waveguide (GW) technology is an emerging and viable contender for the development of radio frequency passive components and antenna systems. The GW technology is based on the parallel-plate waveguide principle. According to Maxwell’s equations, ideally, no wave propagates within a structure if the top plate consists of a perfect electric conductor (PEC) while the bottom plate is a perfect magnetic conductor (PMC), and the distance between the plates is less than a quarter wavelength. This PMC structure creates high surface impedance characteristics which provide cutoff to all propagating modes within the airgap. In practice, these magnetic conductors can be created artificially using an Electromagnetic Band Gap (EBG) structure. Based on this simple principle, considerable research and development of novel components with high performance has seen the light in the past decade. The purpose of this paper is to present an overview of current advancements in the design and technical implementations of GW-based antenna systems and components. Practical applications and the industry interest in the developments of the GW technology for mmWave and sub-THz applications have also been scrutinized.