I am not sure about code level but you can configure after WP85XX module is up.
when you boot WP85XX module with AT command you can find all the GPIO pins that are available
this can be configured according to your requirement. steps as follows.
If the 2nd element is “0” then its unallocated.
Allocate GPIO (example GPIO36) if it is unallocated using AT commands and reset the module:
16 is the configuration that we need to assign for GPIO (example GPIO36) as below,
microcom -E /dev/ttyAT
If the GPIO pin which you are expecting is not listed above then probably that GPIO pin is not enabled.