Problem:
After loading the .gps file into the Trimble FMX, the cut/fill map location is not matching the real world location of the field as I drive around the field. It looks rotated around the master benchmark.  

Solution:
Some FMX firmware versions (maybe V4.01) do not support control grid rotation.

It works in earlier firmware version (V3) and Trimble have fixed this in later firmware versions (Eg V6).
It is recommend you should ideally upgrade the FMX firmware to a version that supports the grid rotation so you get the exact OptiSurface design into the machine.

If this is not possible then exporting out of OptiSurface Designer with a '0' grid rotation should produce satisfactory results.
To do this you set the 'Export Grid Bearing (deg)' to zero when you export the design using File > Export Proposed.
See the screenshot here as an example: