DISM GUI is a graphical interface for the DISM command line utility written in the .NET. DISM GUI allows you to mount and dismount WIMs, manager drivers, features and packages.
Why have those not been moved over to the GitHub repository?
I would posit, that as the author of the software, you should have had a local copy to do this with, without having to rely on the archive export from the CodePlex archive website (now no longer available), even.
The repository has only two commits. Looking at https://web.archive.org/web/20171227212009/http://dismgui.codeplex.com/SourceControl/list/changesets (from before CodePlex went into archive mode), there are more change sets in version control, that more accurately reflect the age and progress of the project.
Why have those not been moved over to the GitHub repository? I would posit, that as the author of the software, you should have had a local copy to do this with, without having to rely on the archive export from the CodePlex archive website (now no longer available), even.