Well, I could not sleep last night and spent some quality time with the Tx and the only manual I could find online.
I have a slightly better sense of how it works, especially what channels connect to which functions on the DMD.
They are different from what Bill had posted, and the Tammy manual.
On my Spektrum the Aileron and Elevator are on the right stick, and Throttle and Rudder are on the left.
Here's what worked:
Receiver - DMD
Bat - unused
1. Thr - M Gun Ch3 (elevation/fire) - left stick
2. Aile - Rudd Ch1 (steering) - right stick
3. Ele - Throt Ch2 (forward/reverse) - right stick
4. Rud - Turret Ch4 (traverse) - left stick
5. Ger - unused
6. Aux - unused

I went through all the settings and wrote them down.
Nothing really jumped out at me in terms of settings for the channel controlling M Gun.
After I reset them back to factory settings, I was able to get all the standard functions to work, including an old Tammy elevation unit.
However, the aftermarket fire button did not work at all, only the stock method worked. (trim down, stick down)

The only way the servo elevation worked is directly attached to the M Gun channel.
But then of course, the gun does not fire.
Which proves Brett's point that the channel needs to be split.

The mystery to me is why the fire button does not work, even in stock setup.
It could have been assigned/wired into channels 5 or 6?

I'll keep playing and give an update.