I don't know how i messed this up but here is the correct link:
https://raw.github.com/gtkd-developers/GtkD/master/src/gtkc/gdktypes.d

OK, I rebuilt with that, and I'm happy to say I can now cut and paste, and my
serialize() function does not crash, so as far as I'm concerned, GdkAtoms fixed.

However, that led me straight into something else. I'll do a separate post.