When you use MP3Gain it writes an APE tag to the file that contains the normalization data.
Mp3tag reads the APE tag first so you can't see the regular ID3v2 information.
In Mp3tag go to File > Options > Tags > Mpeg and uncheck all three APE boxes. Also make sure all three ID3v2 boxes are checked. If you use ID3v1 tags, check those as well.