Source code has moved
The source code for Gum has moved to github:
Documentation will still be housed on CodePlex due to the complexity of adding images to the GitHub wiki.
Gum is a general-purpose UI tool for games, mockups, and websites. Gum saves to an XML format which you can include in your projects, but also supports a plugin system (using MEF) for custom exports.
Gum combines the power of programming concepts such as inheritance, instancing, and default/overriding variable values with a easy-to-use visual editor.
These images were taken from the free sample project here:
How do I use Gum?
For information on how to use Gum, head over to the Documentation