If you survey with other gps gear, then you need to modify the survey data to match the multiplane.txt format. The critical piece is the Lat Long on the first line.
However, see this article here for additional information:
Surveying with Any GPS Equipment and Exporting to a .gps File for the Trimble FMX
We are currently making this simpler in OSDv1.4 of the software due out in weeks.
Import to OptiSurface Designer and export to .gps file now works seamlessly with OSDv1.4 and higher.