JavaMP3Gain is a Graphical User Interface for the command line version of mp3gain (tested with version 1.4.3). The latter is available at this location: http://mp3gain.sourceforge.net/
I have based the interface on the already existing MacMP3Gain: http://homepage.mac.com/beryrinaldo/AudioTron/MacMP3Gain/
To run JavaMP3Gain, place the executable file of mp3gain in the current directory and run JavaMP3Gain as follow using your favorite Java 1.4 or later virtual machine:
<path to java.exe> -jar <path to JavaMP3Gain.jar>
For developers, I used NetBeans 3.6 to develop the source code found in JavaMP3Gain_20040902_3_src.zip.
Second release: comes with a German translation of the GUI from Wolfgang Foerster.
Third release: comes with a French translation of the GUI from Georges Lévêque.