Draft: d/rules, d/girtod: Look for Gst-1.0.gir in ${libdir}/gir-1.0
Otherwise this package could regress if libgstreamer1.0-dev moves the architecture-dependent Gst-1.0.gir to that directory, so that it can return to being Multi-Arch: same.
In fact this doesn't seem to be necessary, because gtk-d doesn't actually run girtod to regenerate its D code at build-time - but I'm sending this untested MR with a potential solution in case it becomes necessary in future.