So I have this brand new ATR-55 mic yet I've tried it in 2 different cameras now and I only get sound out of the left channel. I tried it in my friend's Sony and in my brand new purchased GL2. Audio still only registers in the left channel.
Let's make a tutorial out of your question so others may also learn.
The ATR-55 is a mono mic. The GL2 has a stereo, unbalanced audio input.
I have a problem with any camcorder calling itself "professional" when it doesn't have XLR audio inputs, but I'll save that for another day. Cameras that do have XLR inputs for mic or line level sources frequently have a switch that allows one input to feed both channels.
Your problem isn't the mic, it's the camera's "plumbing" and your expectations.
You expect a mono mic to feed a stereo input across two channels. The camera is not wired that way. (well, maybe it is, but you may not have found the menu options IF they exist.)
So you need a plumbing solution. An adapter that is wired to feed mono audio to both camera inputs. I know of cables that have an XLR on one end and a specially wired 1/8" TRS plug on the other end, but, off hand, I don't know of any simple barrel adapters. Not to say they aren't out there. Call Audio Techncia in Stow , OH and ask to speak to customer service.
When you get to that person, tell them about your plumbing issue. I know AT makes some adapters, I don't know if they make one like this.