This page illustrates how I measured the transmit frequency of a gunnplexer module with just a wavemeter and another gunnplexer module.
Disassemble one gunnplexer module from its mount and remove the feedhorn. Attach this module to either one of the wavemeter's two waveguide ports. Waveguide assemblies and ports only match up in one direction; if you find that it is difficult to line up the screw holes to attach the gunnplexer module to the wavemeter port then you have it aligned in the incorrect direction. If it is not aligned in the correct direction, just rotate it 90 degrees and try again.