unwanted distortion / compression on track, help!

Are you sure your song isn't clipping, either the individual track or the mix? (Clipping means that the meters are going beyond the red region and the two little dots are lighting up.) GB is rather tolerant with this, especially with clipping on the master track, but you'll hear it in the mixdown.

Depends on what's clipping. If it is just one track, turn the volume down. If you think it's too soft then, you might apply some compression or limiting on that track which will allow you to crank up the volume without more clipping.

The same applies to the Master Track: Just lower the volume until the whole song just about doesn't clip any more. If the result doesn't sound "loud" enough compared to commercial recordings, the reason is that those are mastered, which is a complicated combination of compressors and filters that can't be explained in a couple of sentences. What you could try is apply the MultiBandCompressor to the Master Track and pick one of the presets - maybe you'll like the results.

Six tracks ain't nothin'.

First make sure each individual track does not clip. If you need more volume from a particular Track simply double the track (duplicate the track and copy the region(s) to the new track). That way you can get more volume without clipping. If the square light at the end of the volume meter stays lit that means clipping. (Also note that GB's meters are not always 100% accurate. I've had tracks clip on one playback, then not clip on the next).

Then when you're done make sure the Master Volume does not clip.

You may also want to try some sort of compression on the individual tracks and/or on the Master Track. This can bring the overall levels up some without clipping.

There's always the Auto Normalize Preference, but most of us don't like that. It will bring the levels up or down as needed upon exporting. But then you may not get the sound you want. I personally wouldn't use it.

There is no one formula. It takes trial and error and experience. But you should be able to get an undistorted export without much trouble.