A while ago you'll recall that I wrote a little Python script that would average out the sound check gain adjustment created by iTunes for a given set of songs.
I've gone a step farther and created a simple GUI tool for doing the same.
Album Sound Check (download link) provides a basic interface to editing this field based on the values in a given set of files.
After running the program go ahead and open a directory containing songs from your iTunes library. They will be scanned in and the gain dB adjustment values stored. ASC will then determine an average from this.
You can choose to not have certain songs affect the resulting average by unchecking the "Use" column. You can choose to not have the computed average applied to a given file by unchecking the "Apply" column.
Once you're ready just click "Apply" and wait for the spinner to stop. It will then rescan the files showing the new value in place.
There is no undo. Sorry, not something I'm worried about. You can remove the songs from iTunes and re-add, that might work. No promises.