According to this 6 year old thread
it's not really normal to use this format.
To be honest, this is the first time I'm hearing about it.
To find out if the gencad output is correct you either need to find someone who knows the format and can look over that file or a tool that can load it and validate it.
Maybe you can try these (seem to offer evaluation versions for testing):
I'm not affiliated with any of this, so you're on your own.
PS: you might ask the fab what they need this format for - maybe there is a more common format that KiCAD can provide that does the same job?
PPS: I just tried GenCAD export in a KiCAD nightly on Win7/64bit (Application: kicad
Version: (2017-02-10 revision 1bcbbb4)-makepkg) and didn't had any problems. All my symbols & footprints are self-made though.