They used a spherical resolver (metalic cricket ball on a flat disc) to computate travel and position, and because most of the SAN navex routes departed Sale via Mt Livingstone (60 miles north east?), over time, they developed a small groove where the metal sphere travelled on the plate. This meant that no matter where you departed to (in a northerly direction) the GPI ALWAYS went to Mt Livingstone. Good start for a trainee nav cadet who was lost shortly after take-off. It took three to four '20 minute' nav cycles to get the 'plot' back, by which time you had erroneously entered Sydney Control airspace somewhere east of Parkes on your way to St George.
If they were fitted to Vulcans, how the bloody thing got them to Stanley is beyond me.