Fedora Linux 17 using google-chrome-stable.i386.
1-2 months ago GM_xmlhttpRequest stopped working in google-chrome-beta.i386 for Fedora 17. Response was "Uncaught ReferenceError: GM_xmlhttpRequest is not defined".
This seems to be going from version 26 to version 27 of Chrome.
Solution at the time was to remove beta and install stable as apparently beta version was 27, stable was 26.
On latest update today for chrome-stable this problem reoccurs. Stable version yesterday was 26.0.1410.63. Stable version today is 27.0.1453.93.
Script in question which runs GM_xmlhttpRequest is a .user.js script installed manually into Extensions.