Hell Fortress: "I have some business on the other side" dialog should let tux 
through the gate when the disruptor shield is down.

I gave up after being told to go away by both guards. Only to come back and 
realize that I needed to select "I would like to go through the gate" instead of 
"I have some business on the other side"

Side tangent: why doesn't the gate go down automatically when the disruptor 
shield is disabled?

The gate which is open is the original from the factory, the gate which is closed, 
was placed by the Red Guard as part of their efforts. If you coerce Bruce, he will 
explain a bit on how they put this additional gate, but you'll miss out an 
optional quest if you do.

I guess I need to look at the dialog script.

I did not get assigned the optional quest despite exhausting the dialog tree.

Optional quest is assigned by Dixon 
iirc. You must bring Bruce the helmet 
to continue, but this is disabled if 
you coerce him.