I want to automate the levels of gain in a track?  I want to use gain rather than volume because I want to still be able to mix the overall volume level with the volume slider.  Or do I have to automate the volume and use the gain knob to mix the overall track level?   Or something else ..?

Not track gain AFAIK. You can automate gain on a per clip basis.
 
Once volume automation is written it is easy to adjust overall level either via Offset mode or adjusting the whole envelope by click dragging toward the top third of the envelope lane with nothing selected (Ctrl+Shift+A) or using the gain control but the gain control is really there for proper gain staging rather than mixing purposes.

You could add an empty FX chain in the FX bin or ProChannel if you want an additional automated gain control.

scook
You could add an empty FX chain in the FX bin or ProChannel if you want an additional automated gain control.

Great idea!

Sidechained compression may be something to look into.

ULTRABRA
...automate the volume and use the gain knob to mix the overall track level?

This is how I usually do it.
 
The downside is that if you have a compressor on the track, changing the gain requires adjusting the compressor threshold.
 
In that situation, there are two options for tweaking overall track volume after automation:
- Use the compressor's output level control, or the the output level control of the last plugin in the fx chain
- Freeze the track effects