Seats are interchangeable from first GS in 2004 through to the last of of the hexheads in 2013.
The GS and GSA are the same seat as well.
Just be aware that there are low and standard heights of seats when looking for one.
I see it finally went for £200.
While that is a huge saving over the price when new it should be possible to get one for around £175 on the 'for sale / wanted' section on this forum.
Prices seem to be slipping on accessories for the hexhead.
I recently bought a low version of the Sargent seat in excellent condition recently for £150.