Gas Heat Pumps (GHP) are an emerging class of high efficiency heating equipment that upgrade low-quality ambient heat via an ammonia-vapor absorption process. This technology can reduce fuel consumption of space heating systems served by furnaces or boilers by 35%-55%, with rated seasonal heating efficiencies of 140% AFUE or greater, based on the application and climate region. GHPs can also provide cooling by employing a reversing valve, albeit at a lower Coefficient of Performance (COP). GHPs are commercially available, but the commercial space heating and cooling market adoption is low. A September 2022 report by the IEA calls for further policy support and technical innovation for heat pump systems to reduce upfront purchase and install costs, remove market barriers to complex renovations and to improve equipment performance and durability for power system integration and flexibility. When integrating with a legacy system, a hybrid plant with GHPs also meets energy resilience goals through continued operation during electric grid outages and can present a hedge against future electricity and gas price increases. The authors explore the sizing, design and economics of a novel hybrid boiler and chiller plant with an integrated reversible GHP array at a commercial DoD office building in ASHRAE Climate Zone 5. The goals include peak electric demand reduction in summer and gas therms savings in winter while maintaining energy resiliency goals. Energy used for space conditioning was measured over the 2022 and 2023 baseline period, with utility bill analysis to determine the cost impact of electric demand. The retrofit GHP array will be sized to handle baseload hydronic cooling, with supplemental cooling provided by the existing air-cooled electric chiller. Meanwhile, the boiler plant will operate on an outdoor air temperature (OAT) reset schedule to stage between the retrofit GHPs (during milder heating season) and the existing 83% TE single stage non-condensing boilers (for peak heating demand). Critical design parameters not limited to the return fluid temperature influence on GHP operation, capacity deration due to glycol use, will also be discussed to provide readers with a perspective on effectively sizing and integrating hybrid heat pump plants to achieve application specific goals.