Dano is right, there seems to be no way to do this work from inside of Mp3tag due to a possible bug in the hierarchical tag control system.
So you have to use an export script like this:
(set Options Mpeg 'Read APE')
$filename(Export.MP3Gain.txt,ANSI)$loop(%_path%)^|%_PATH%|%REPLAYGAIN_TRACK_GAIN%|%REPLAYGAIN_TRACK_PEAK%|%REPLAYGAIN_ALBUM_GAIN%|%REPLAYGAIN_ALBUM_PEAK%|%MP3GAIN_MINMAX%|%MP3GAIN_ALBUM_MINMAX%|%MP3GAIN_UNDO%|$
$loopend()
Export_APE_MP3Gain_fields.mte (224 Bytes)
For importing you need a proper format string for the converter function 'Textfile to Tag':
(no APE involved, unset Options Mpeg 'Read APE')
^|%_PATH%|%REPLAYGAIN_TRACK_GAIN%|%REPLAYGAIN_TRACK_PEAK%|%REPLAYGAIN_ALBUM_GAIN%|%REPLAYGAIN_ALBUM_PEAK%|%MP3GAIN_MINMAX%|%MP3GAIN_ALBUM_MINMAX%|%MP3GAIN_UNDO%|$
Afterwards if all went fine you can delete the original APE tags.
Be aware that MP3Gain 'touches' the audio data in such a hard way (it manipulates each MP3 audio frame's volume setting), so that only MP3Gain is able to reverse the audio stream back to its original status, if you sometime want to restore the true loudness curve as delivered by the artist.
Moving MP3Gain tag values to ID3V2 and deleting APE tags let you alone with an unrecoverable loudness corrected mp3 audio file.
For all cases it is better not to use the MP3Gain application (beside this the MP3Gain idea is really great, thanks to David Robinson with a twinkle of my eyes). There are alternative applications on the market that support replay gain method too, e.g. WinAmp and especially Foobar2000, which do not change the mp3 audio data itself.
DD.20070317.0844
Moderation: Please use CODEBOX tags for long code lines. 
Export_APE_MP3Gain_fields.mte (224 Bytes)