If I understand MP3Gain correctly, it works by writing a value to the gain header of each block of the MP3. I have a bunch of MP3s that have been MP3Gained, but I used the ignore tags functions so that the information would still display on foobar2000. Now, I'm looking to see if I can bring these files back to their original volume, but I don't have the undo tags for them. Is it possible to merely remove the gain header value of each block to do this? Is there a program to do this? Or does this gain header block hold more information than I'm aware of?

The way MP3Gain works is to change the global gain value of each frame of an MP3. You cannot remove that value.

However, if you are lucky you might find a file called Changes.log or similar (I don't know if this file is written by default) in MP3Gain's program directory. This file lists every MP3 that was changed. With these values you should be able to undo the changes manually.