To use them, just check if the External Tools plugin is activated and copy&paste the tools you want into the Tools → Manage External Tools… menu.
Alternatively, you can download or clone all files and copy them to
~/.gnome2/gedit/tools. They are automatically recognised by the plugin. Make sure to adjust the shortcuts.
Feel free to add useful and handy tool snippets ;)
For additional information and comments, see this blog post