I used a chain object and found a unit was able to pass through the small end cap despite there being no actual gap.
They also seem to be treated like an open gate by units, which causes some pathfinding problems.
Enclose a unit in fences, remove two fences to create a chain-sized slot facing north-west, and place the chain there. In testing, try to move the unit outside of the enclosure, on the opposite side of the chain.
Open a second hole in the fence on the south west side, and move the chain there, keeping the original hole open. In testing, try to move the unit just past the chain, and he’ll ignore the opening.
Chain objects are given the same Blockage class as gates, despite not being able to function as one. This perhaps means the pathfinding is informed that this is a gate, and not informed that it is locked.