Tempted to mark this as "No Access" but for now I marked the gate as "requires permit" which theoretically means the gate is blocking the road unless you have a permit...
Of course this requires that the routing software understands this, so it's up to the software to know that it's not allowed to route through without the permit.

On another note I wonder why this is such an issue if the other entrance is always open... baffles me why it's simply made difficult to access the area...