There is an Ogg Vorbis equivalent to MP3Gain called
VorbisGain, but it works a bit differently by instead adding a ReplayGain tag to the Ogg file which indicates at which level to playback the file. Unfortunately, it requires that the player recognize ReplayGain tags and I am unsure whether RPG Maker's game player/editor does this. You'd have to check for yourself or wait for someone to report back, as I am unable to test this currently.
If you decide to re-encode the file to adjust the volume levels, you're going to experience quality loss due to the file format being a lossy format. If you have access to the recording in a lossless format such as WAV/FLAC/ALAC/etc., you'd be better off using that as the source to then encode into a new Ogg file with the desired changes made from that.
If you're looking for a simple way to convert media file formats, I recommend
xrecode II. If you're looking for any extensive audio enhancements, then as recommend before me,
Audacity is a fine free and open source software. Other alternatives to Audacity may be found
here, or
here for free alternatives only.