I am using a GL2, and am filming an educational video which will have much of the footage voiced over with narration. Because of this, I filmed the footage so far (2 days worth) with the mic adapter connected to the mic input on the camera, but I did not connect the mic to the adapter (being a complete newbie, I thought for some reason this would be better than having sound generated from the GL2's mic). My question is, with what I have done so far, will I still be able to add the narration (i am using Sony Vegas)? Second question - is it better to film the way I filmed so far, or film with the sound from the GL2's mic?

It kind of depends on your script and how you're putting your film together. Since there's nothing standing in the way of muting or completely discarding the original soundtrack recorded with the video once you're editing in post, from a purely technical perspective it doesn't really matter one way or the other. Any sound recorded with the video on the scene doesn't have to remain in the final production. But from an editorial standpoint, it would be very unnatural sounding to show, say for example, a bus terminal with people coming and going, walking close past the camera obviously talking to a companion, buses pulling in an out, etc, and have the sound track carrying JUST narrration without any of the ambient sounds of the busy terminal in the background. So from that standpoint, I'd leave the mic connected and "hot" and try to get a good clean recording of the scene ambience, just to have it on hand to use later.

I concur with Steve, always record with at least one mic connected to capture the ambient sound. BTW, some producers call this "Nat sound". On the final mix if you do not want the ambient sound, turn the track or clip/event's volume down. Any narration is normally recorded in a controlled environment and added later anyway. Also very easy w/Vegas.