It would be awesome to have this, but with the way this command works in order for us to have access to /gm 3 we would also be given access to /gm 1, creative mode. Which we are not allowed to have access to, and could be easily abused.
I am sure there is a plugin available that allows per-gamemode permissions. I am pretty sure it's possible to do within the bukkit file :/ Anyway, /gm 1 could easily just be disabled.
Support! I have seen this happen...a hacker just stops what they are doing then trys to be nice when a mod tp's to them so they won't get banned. /gm 3 would be very helpful to the mods!
Actually, no, creative mode can easily be PeX'ed out of commands that moderators could use. It's literally as simple as /Pex enable "/gm3" Moderator.