I would like to use DDD to debug the source code of Gedit and GTK+. I start DDD by command ddd gedit. Now I can run the project and I found the source window but I can't see the sources of Gedit and GTK+ nor run the program line by line. How to do that?
You have to recompile your sources adding debug symbols.
Look in the documentation of gedit to see how you can compile with them. There should be a Developers or Hackers section that explains how to do that.