On the 1000's the limiting factor is that the shocks are eye-and-clevis. Most shocks are eye-and-eye so you should be able to come up with just about any length you want. You may have to change rubber bushings to fit the bike.
If you take some measurements from the shock mounts and hit a swap meet you should be able to come up with something off maybe one of the lighter big twins.
I would think that shocks off an Ultra Classic or other big bagger would be a bit much for the lighter GL.
2006 GL1800 (Brutus Maximus)
1978 GL1000 (White Trash)"My Effort"
The White Trash Album
....Sharing some thoughts...."Venting GL1000 Rotors"
Hole templates for front and rear (45 holes) and how to use them"Side Stand Indicator Light"
For those of us who can't remember..."GL1000 RearSets"
Photos, CAD drawings and instructions (click "No Thumb" and then "Download Doc")I'd like to apologize ahead of time.