I'm no Guzzi expert, but I have a soft spot for the Griso, despite that monstrous pipe. Having chatted with owners waiting around at the dealers in the past - all said that their idle stepper motor was the problem. BTW, try to resist fiddling with any adjustments until the issue has been researched.
It is fed by a hose from the airbox - if you clamp off the hose, or disconnect & plug it off, the issue should go away. This simply serves to prove the SM is the cause & is somehow creating a high idle scenario - whether it can be repaired I know not; you would need to talk to a dealer/specialist or see what you can find through Dr. Google. I gather, when the air feed is closed off, engine braking will seem more extreme as the motor returns to idle more quickly. Doubtless emissions will be impacted too, but this is only in the research stage.
If it's not the SM bleeding in air when hot, then I can only suggest that you check out the cables/linkages as well as the caps on the TBs for potential air leaks.
Good luck sorting it - did you buy privately, or is there some warranty, implied or otherwise to help.