Not Pb, but Gb interface
One PDCH can carry a certain useful throughput, which can vary from 6kb/s up to 60kb/s.
One GCH on the Gb interface can carry only 16kb/s !
Therefore, One PDCH will need more than one GCH if the PDCH throughput is above 16kb/s (or a bit less than 16kbps actually, due to signalling).
For instance, at 60kb/s (59.2kb/s, more exactly), one PDCH will need 4.5 GCH.