ReplayGain is a Python script that can be used to apply or view Replay Gain information of audio files, like MP3. It is a good replacement for the MP3Gain tool, which has been discontinued since Ubuntu 15.04.
$ sudo apt install python-rgain
By default, ReplayGain applies album replay gain to the input files:
$ replaygain *.mp3
Tried with: ReplayGain 1.3.3 and Ubuntu 15.04