I am using track gain for my Monkey's audio files in foobar.

Whenever I rip a new album and want to replay gain it, do I have to include the older files so that the track gain is the same, or just do the new files?

If you're just using the Track Gain, the gain values for other files are unused and irrelevant, so just the new files is fine. Even if you're processing a new album using Album Gain, you only need the tracks for the new album, ReplayGain always uses the same reference value.