Some files or parts of files are generated from templates to reduce the amount of boilerplate code that needs to be written.
Simply run the GenerateCodeC.py script to regenerate all such files. The output is written directly to the src and include directories from the root folder. This is already done during development, so you only need to generate code when making changes to CTGUI, you don't need it when simply using CTGUI in your project.
All files in src/CTGUI/Renderers/, include/CTGUI/Renderers/, src/CTGUI/Widgets/ and include/CTGUI/Widgets/ are generated based on the .desc files found in the templates subfolder. For some classes there are additional .extra.h and .extra.c files from which the contents is copied directly to the header and source files and placed below the generated code. Additionally some files in src/CTGUI/ and include/CTGUI/ are partially generated. For these files there is a .cpp and .h file that have placeholders for where the code generated based on the .desc file should be placed.
The ValidateDescFiles.py scripts attempts to detect some issues with the description files by comparing them to the c++ files. It can sometimes detect issues which would not be detected by simply building the generated files, such as missing renderer properties.