I'd heard that.
QUOTE(By Snelg)
There's no way to make MP3Gain properly process MP3pro files.
I had not thought about making it skip them, though (the same way it skips mp1 and mp2 files by default).
I'll look into that somtime later...
QUOTE(By Ivan Dimkovic )
Envelope data (energy value) is coded implicitly in the SBR bitstream, and without SBR specs for mp3Pro it is impossible to change the gain of the envelope data.