I've read that Audacity can also do MP3Gain's job to normalize multiple tracks, but while in MP3Gain the target volume is something like 89.0 dB or 95.0 dB in Audacity the maximum is 0 and it goes into negatives, how do I convert between the two values? Are they even the same thing?
dB is just a ratio. 0dB just means the reference volume - ie the output without any attenuation. So in Audacity, you can make the output quieter (ie below 0dB) but that is its maximum.
It is a logarithmic scale, so increasing by 3dB just means doubling the power, and increasing by 10dB means multiplying the power by 10.
Neither mp3gain, nor Audacity refer to output volume, but simply the amount they amplify or attenuate the power. So on your system you can find out which numbers are equivalent, but those might be different to mine.