Greasemonkey version 1.13 is in the works, with Sync support for installed user scripts. Support is currently very basic, but we'd love to hear feedback!
It's suggested that before you try Sync, you take a backup of your scripts directory. Then open the Greasemonkey options dialog and enable the experimental feature, which is currently disabled by default:
For now you'll need to install version 1.13beta1 to have access to this feature. Please provide feedback at the greasemonkey-dev mailing list or the GitHub issue, good or bad.