I don't believe there are any disk images that "are known to work perfectly" with gem5 or gem5-gpu. As the simulator changes and evolves, driver and library version constraints can be changed/introduced that upset whether Linux versions will boot and whether benchmarks will run. However, as far as I know, you should be able to boot with, mount, and explore most/all disk images that are publicly available. A lot of this involves trial and error, including booting Linux, and then connecting to the simulated system with a terminal to see what commands work, etc. Many of us have built our own disk images and have modified the way the simulator uses them.
That said, I believe that both Linux 184.108.40.206 (non-SMP), and 220.127.116.11 should boot with x86 in both gem5 and gem5-gpu. The disk images available on the PARSEC M5 site should work to allow you to mount and look around on the disk. You should look at the system.pc.com_1.terminal output to verify that hda and/or hdb are recognized during the boot process.
To connect a terminal to the running system, just build m5term (which I pointed you to previously), and run it with the command line parameter port that the simulator prints (usually 3456 for a terminal connection).
Finally, I would strongly encourage that you spend more time searching the gem5 email list archives
for your questions. Basically all of the questions you've asked here are answered in the archives, and if you can find it there, you won't have to wait for us to respond.
Hope this helps,