Glide-symmetric holey electromagnetic bandgap (EBG) structures have found wide applications in high-frequency gap waveguide components because of their demonstrated wide stopband and easy manufacturing. However, potential dips in the transmission through the gap waveguide at certain frequencies limit the effective bandwidth. Here, we perform a Bloch analysis of the unit cell, a rectangular waveguide segment implemented with the glide-symmetric holey EBG, using a multimodal transfer matrix method. We find two main spurious dips in the transmission coefficient in the recommended operating frequency band of the investigated WR-15 standard rectangular waveguide. The first transmission dip is found to correspond to the coupling of the waveguide mode and the edge mode formed in the air gap between the waveguide and the EBG holes. The second transmission dip is caused by a small open stopband in the waveguide mode.