Put the scripts in the same folder as the gtk.gresource file. Make the scripts executable. Run extract.sh. See if the png's are able to be opened. If not, you will need to replace them with the original assets. Try to find them on github or something. Make sure the assets aren't in an /assets folder in the gtk.gresource file. If they are you will need to edit the script to put them there, or edit the css files so they grab the assets from the root dir. Run build. That's it.
JohnOberhauser/GResourceScripts
Script to extract and build gresource files