By GPIO 18, did you means, the Pin 18 of the 9210 module?
If yes, then it correspond to the GPIO 9 (or SCL for I2C):
cme9210 Connector Pin 18 ==> GPIO 9 (SCL).
GPIOs pins handled by the system are declared in the file "etc/makedev.d/cme9210js" (or some name like that). You should check this file. If it contains the GPIO 9 entry, and you have load the "gpio" driver, then this GPIO cannot be use by another driver than the "gpio" one. Hope this help you.