Due to changes to both, GameMaker and Oracle VM software, I can no longer reliably test my games under Linux. I am in process of setting up a real linux box, but it's proving a lot more difficult and time-consuming than I expected. I can, ofcourse, just upload an older Linux build here, but that's not something I like doing.
Have you tried QEMU (http://wiki.qemu.org/Links)? I think it's much better than VirtualBox on Linux, but I don't have any experience with the unofficial Windows version.