This is just to help remember:
If you have used G_DECLARE_* macros in a C class header in order to modernize your old C code to resent GLib class definition methods; you are using GObject Introspection to parse it and generate GIR files; and you get this error:
"Fatal: Gda: Namespace conflict for 'your_type_get_type'"
Then remove your “*_get_type()” method definition from your header file.
Above have been the result to modernize GDA to recent practices and could be helpful for any one else.