If this website hasn’t answered all of your questions yet, here are a few more resources:
Configuration and tweaks
We strive to make GemRB usable out of the box, with as few settings as possible. That belies its configurability, so here is an assortment of pages, mostly related to customization:
GemRB is a big piece of software with many subsystems and tunables. Check the dedicated developer documentation page for an overview and some specifics.
Additionally check out IESDP, a project that is coevolving with GemRB and stays immensely useful. It has a nearly complete and up to date documentation of the (community’s understanding of the) Infinity Engine.