This behavior is by design. For steady state, the transient term is absent, so there is no buildup of pressure. For steady state incompressible flow cases, when there are no pressure boundaries, the gauge pressure value at each cell is subtracted from the gauge pressure value at reference pressure location. Thus, absolute pressure is operating pressure, plus gauge pressure, less absolute pressure at the reference pressure location. Hence, whatever gauge pressure you prescribe or patch immediately cancels out. Initial density for compressible phases is calculated based on temperature and operating pressure. There are two ways to represent steady state enclosure conditions at a certain pressure: either run steady state with operating pressure equal to initial absolute pressure, or run transient, in which case the solver will let you patch gauge pressure without zeroing it out.