If gnome has hung then forcing gdm to stop is as brutal as init 3 and init 5
In my opinion it is cleaner as well.
I have noticed say if I am opening an SVG file (5mb or so) in gimp (gimp can import SVG only), it would sit there at 100% utilisation for file-svg process and never import it. While that was happening I had reloaded the shell with alt + f2 r and one of my extensions crapped itself and it presented me with a logout option.
Once I had logged back in the file-svg was still running at 100% as was a nautilus process. I thought hmmm a logout should have killed both those processes (as they were running under my normal user account), I logged out again and back in, and the file-svg was still running and now I had 2 nautilus processes running at 100%
Long story short - if you want to be sure you are killing any rogue processes or make sure a gdm restart has unloaded and cleared all modules 100% an init 3 / init 5 is the way to do it.
But that's from one noob to another