Glade is a RAD (Rapid Application Development) environment of sorts similar to Borland Delphi or Lazarus except it handles only the windows and widgets, there's no complete IDE. You use your favorite editor to write C or C++ code (or maybe other things too). The application you create is saved as an XML file plus your C, when you link it with libglade at compile time libglade replaces the XML with standard GTK widgets at runtime. The executable does have libglade as a dependency.
It doesn't write the code for you but if you've ever looked into writing GTK this is a big timesaver. There's a more complicated tutorial I gave up on several times, this is a single page that walks you through creating a simple window. But be warned that devhelp doesn't work on a Pi, you'll have to use the online version. Actually if you look in /usr/share/gtk-doc/html you'll find most of what devhelp will show you, it would be simple to make yourself an index.html file at that level that links to the index.html files in each section. I lived with dialup for years so I got used to tricks like that.
The simple application which does nothing actually:
The tutorial has some templates on another page for later use, but start at http://prognotes.net/2015/06/gtk-3-c-pr ... g-glade-3/. Some programming experience is advisable, but I've been trying for years to create anything usable with a GUI under Linux. It's easier to write CGI or stick to command line.