This article overviews emerging state-of-the-art GaN technologies for next-generation millimeter-wave applications. A brief discussion of millimeter-wave applications for high electron mobility transistor (HEMT) and monolithic microwave integrated circuits (MMICs) from the Ka band (26.5-40 GHz) up to the D-band (110-170 GHz) includes front end, switching and PA applications. Graded channel GaN HEMT, AlN GaN, HEMT, ScAlN GaN transistor, and N-polar GaN device technologies are described for various applications. Scaling and epitaxial/device codesign approaches and doping considerations are discussed to improve device performance. Finally, the importance of thermal management to assure reliability is addressed as devices get smaller and faster - and hotter as a consequence.