MC can automatically analyze the ripped files for replay gain. The option is here:
Options > CD & DVD > CD Ripping > Analyze audio during ripping
The file converter copies the replay gain values to the converted files automatically.
If you have unanalyzed files you can analyze them manually with a separate tool:
Select the files > Right-click > Library Tools > Analyze Audio...
The playback replay gain options are in DSP studio > Volume Leveling (check the program help for more info).
Also, MC can automatically write the iPod soundcheck value when it transfers files to iPods if the files are analyzed with MC.