Many general purpose GPS devices utilise NMEA protocol, which is a standard protocol for GPS devices. However, the problem with using these devices for as a GPS Clock for computer timing is that the NMEA sentances are often not synchronised to be generated at specific intervals.
Many GPS devices therefore can only provide an accuracy of plus or minus 1 second, which is generally just not accurate enough for network timing purposes.
Some GPS devices also have PPS (pulse per second) outputs to provide a higher degree of accuracy. However, again quite often the NMEA gps sentance is not synchronised to the pulse output providing poor accuracy. Also generally the PPS output is provided in a TTL (5V) signal level rather than a RS232 signal level that can be recognised by PC equipment.
Galleon Systems TS-900-GPS and TS-700-GPS GPS Clock devices utilise an accurate binary protocol synchronised with a highly accurate timing pulse. This allows for a much more stable and highly accurate timing solution.