I recently picked up a 1980 GS550. I joined the GSResources forum and it's pretty cool, but not quite as active here and a little bit more focused on the larger bikes.
The '80 seems to be something of a transition year. Many parts swap with the '77-79 models. I'm trying to cross reference parts and numbers, but a parts fiche doesn't always tell you if something will or won't work.
For example:
The '77-79 models used a head gasket and four separate sealing rings. The '80 shows a different part number and it looks like the sealing rings are now part of the head gasket. I see no reason why the earlier (and much easier to find) gasket and sealing rings wouldn't fit. But I'd like to be sure.
Parts availability for the GS series is OK, but not quite as good as the Honda stuff. And exhaust options to the '77-79, '80 550 is basically used stock or MAC.
They seem to be good bikes, though. It's handsome, the engine is supposed to be very sturdy and dependable and good for 100K, and the chassis got good reviews for handling in period tests.