Trying to make a gate at the end of a deck (full width)

[mctabish]

This is actually a bridge, and I am trying to put a gate (12ft wide) at the end.
However, I cannot make the gate wider than half way across the bridge.
I am using x4.

I made the deck, I added a rail all the way across and then inserted a door, I tried to make it wider, but could not


Thanks!
Mc