Is there a repository where we can find archived versions of grasshopper installers?
Can we install multiple versions and toggle which version to run?
If you had a way to swap out the installed files, then yes. You could for example set up a git project and install different versions in different branches. But then you could only switch between Rhino sessions.
I’m assuming you’re looking to do this for Rhino5, as gh is shipped as part of Rhino6.