  \textit{Resolve} onboard the X-ray satellite XRISM is a cryogenic instrument
with an X-ray microcalorimeter in a Dewar. A lid partially transparent to
X-rays (called gate valve, or GV) is installed at the top of the Dewar along
the optical axis. Because observations will be made through the GV for the
first few months, the X-ray transmission calibration of the GV is crucial for
initial scientific outcomes. We present the results of our ground calibration
campaign of the GV, which is composed of a Be window and a stainless steel
mesh. For the stainless steel mesh, we measured its transmission using the
X-ray beamline at ISAS. For the Be window, we used synchrotron facilities to
measure the transmission and modeled the data with (i) photoelectric absorption
and incoherent scattering of Be, (ii) photoelectric absorption of contaminants,
and (iii) coherent scattering of Be changing at specific energies. We discuss
the physical interpretation of the transmission discontinuity caused by the
Bragg diffraction in poly-crystal Be, which we incorporated into our
transmission phenomenological model. We present the X-ray diffraction
measurement on the sample to support our interpretation. The measurements and
the constructed model meet the calibration requirements of the GV. We also
performed a spectral fitting of the Crab nebula observed with Hitomi SXS and
confirmed improvements of the model parameters.
