The comment is not really much clearer. It explains how Walter's documention of ImportC fails to explain that it can handle header files for GTK.

But in terms of how you actually do that it says:

I simply changed my makefile for this application to, in the invocations of dmd that compile my D source files, include the standard pkg-conf call that expands primarily into '-I's, indicating where to find all the gtk header files.

Thats not clear at all. I know where the GTK headers are, how do I tell the compiler? I tried make a file C file which included gtk, the compiler tries to import it and says

C preprocessor directive ‘#include’ is not supported

Which directly contradicts what is said in both the original post and this one.