The gex geometry format is described in the attached documents for SkyTEM skb and SkyTEM xyz.
There is also an attached document going over the how the timings in Aarhus Workbench are calculated from the geometry file. It also covers a few related details.
Two other geometry file format exist.
The ge2 geometry file format. This is a variant of the gex geometry file that leaves out the parameters already present in the (.skb) data or (_PAPC.sps) navigation files for the survey. Instead those parameters are read from the data and navigation files during the import. As this import also creates a gex geometry file with the complete set of parameters, most will only ever see the gex version of the geometry file.
The geo geometry file format. This was the previous version of the geometry file. It is still possible to import data with this legacy format. Doing so, will automatically create a gex geometry file of the same geometry information.