I use a Mac, and I have done exactly what you suggest (on a Mac though, the gpx executable is inside the app itself, not in a directory). But yes, it works.
If I remember correctly, it was only passing the -m option to specify the machine, as well as a file name.
I am not sure, but I think that the version they bundle is one built from this GitHub repo
https://github.com/whpthomas/GPX .
There is a newer fork which fixes some bugs (in particular, the one that was bothering me was that gpx that simplify3d bundle is capping the HBP temperature at 120, not the 130 that the firmware uses) -
https://github.com/markwal/GPX - this version is working well for me.
If you do use a Mac then I can help you out with a binary if you don't want to compile it yourself.
Cheers
Slip