GPRS Call flows can be found in 3GPP specifications. Lots of information via the usual Google search.
Depending on the features that are activated a GGSN can have the capability to act as a Router, Firewall and a Gateway. To confirm the end user’s details it can send RADIUS Packet Data requests to a AAA node.
With 3G and 4G there is now the notion of a multi-functional APN that points to a basket of Services / bundles that can be consumed.
VBR/ Wallis Dudhnath