There are things about the G5 that would lend itself to low-light concert recording, and others that make it hard.
The lens is fast- f2 at the wide end and f3 at the long end, but you need to be fairly close. 140mm doesn't get it done from much of a distance. The images get pretty noisy at ISO 400, which is where you'd want to be. The plug-in Neat Image helps with that alot. Maybe tele attachments would help get closer (you'd still have an f3.0 tele lens as the attachments don't "slow" the lens down). The G5's spot metering is very handy, but you may be dealing with some serious purple fringing issues where the dark borders and white lights meet, which would mean extra work in Photoshop to correct. I've had low-light focusing problems with my G5- enough where I've created two zone focus settings with the two available custom settings on the dial, but that's harder to do at the tele end than at the wide end since DOF is shallower. In the end it may be your only option, security wise, unless you're part of the media. A Digital SLR would be the best choice with cleaner images at higher ISO ratings and the option to mount better lenses (but G- series lenses are excellent), but security might not let you in.
The G5 has a great macro focusing system. You can get pretty darn close without and supplementary lenses, but once you switch into macro mode, you have to switch back to regular shooting modes when you're finished or the camera won't focus at regular distances.