A high equivalent isotropic radiated power (EIRP) active phased array antenna system has been designed and experimentally verified at the 28 GHz band. The phased array employs Gallium Nitride (GaN) based radio frequency front-ends with 31 dBm output power in transmit mode and 3.5 dB noise figure in receive mode. A fully metallic gapwaveguide technology has been employed in order to achieve an efficient heat dissipation per aperture area of the array as well as low-loss array antenna elements that are easily manufactured. The phased array is realized by sub-arraying an 8×8 slot array antenna with horizontal polarization. The presented antenna system is capable of analog beamforming in the range of ±60 degrees in E-plane. The presented high-bandwidth phased array antenna system is a potential candidate for high power and compact size 5G base station antennas for wireless communications requiring high temperature stability at the millimeterwave bands.