Can someone explain to me the difference between retro GT frame models?

[synchronicity]

Which ones had double butted tubing for example?

 

[legrandefromage]

All the catalogues are there to browse and compare models

Just click on the archive link

 

[kingoffootball]

There's some more info in this thread that might interest you, along with the catalogues:

https://www.retrobike.co.uk/threads/gt-heirarchy.102251/