Bump, just built the GG for myself and made sure to place it between two non cliffside hexes, found something interesting, the cliffs in your entre hex are part of the bridges graphics not a cliff face for that hex.
If you're still able to zoom in on the bridge and you will see what I mean.
This is still a bug, as you should not have been able to build the GG if the cliffs were going to block movement. However the pathing is correct and is not a bug in this case.