Not GCH, but Frame Relay packets:)
The structure of Gb-interface is another than Abis. There is Frame Relay Protocol stack on it. It is a packet switching technology. The Gb-interface is realized by PCM lines. The Gb-interface phisical layer is called Frame Relay Link (FRL). FRL is a nx64 kbit/s phisical channels, created oher a PCM-line. Each FRL carries the packet switched data traffic of a specific number of cells.