I'd like to add that you should be sure to listen to the audio *at the camera*. Get a good pair of headphones (Sony 7506 or Senn 280 are the board favorites). Just because the meters on the GL2 aren't clipping doesn't mean that the audio isn't (trust me, I made that mistake... once).

I usually stack two 20db pads (total of 40db on one channel) on my GL2 when using a mixer feed. I may or may not use the built-in attenuator. Also, as far as I know the only input in camera mode is the mic mini-jack (I think the A/V jack on the back is an output in this mode).