Both GPON and GEPON have built-in bandwidth allocation features in their traffic delivery mechanisms. This means that if a subscriber is hogging a lot of data, they can get extra bandwidth that isn't being used by others at that moment. This is clearly some pretty complex tech, so I won't pretend to know exactly how it works under the hood. Long story short, with a 1 Gbps downstream channel, a single subscriber could actually get the full speed—assuming the ISP allows it and nobody else is online at the time. The bandwidth is shared among all active users, and individual speeds only start dropping if the total capacity gets completely maxed out. Just to recap, the bandwidth can be capped either on the ISP's configuration side or directly by the PON hardware itself.
This article is a translation of the original Russian-language post.My journey of learning GPON