From an external network's point of view, the GGSN is a router to a "sub-network", because the GGSN ‘hides’ the GPRS infrastructure from the external network.
从外部网络的角度来看，GGSN是一个到某个“子网”（sub-network）的路由器（router），因为GGSN对外部网络“隐藏”了GPRS基础设施（infrastructure）。