I am unable to get a herringbone gear like in abdullah's first post. In fact, I can't even get a single helical involute gear. It should be possible according to the FCGear repo's README. How is it supposed to work?
Changing the double_helix parameter from false to true does nothing.
These are all the properties listed.

- FCGear_Involute_Properties_01.png (50.43 KiB) Viewed 701 times
I've updated FCGear, relaunched FreeCAD and tried FC v0.17.13541 as well as
OS: Ubuntu 18.04.1 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.14575 (Git)
Build type: Release
Branch: master
Hash: a624fee2c27bbbb869df6a8ce40438953a282a8c
Python version: 2.7.15rc1
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: French/Canada (fr_CA)