The majority of the config can be found (in the appliance) at /usr/share/grommunio-common/nginx/*
There are for example the log file definitions you are referring to. However, please note that any changes there should be taken careful into consideration, as these configurations are deployed automatically by the package grommunio-common. This package is also regularly updated and hence means that your changes there could be overwritten.
The entire configuration is loaded via the major hook at /etc/nginx/conf.d/grommunio.conf (line 4)
If you are intending to replace these defaults, it could be wise to keep your configuration monitored based on updates you make, since they have been placed by grommunio to simplify the installation for the majority of the customers. Of course, by using kubernetes/docker, or similar you'd not be using grommunio-default but instead deploy your own configuration trees based on what we ship.