I can't find where to go after I find Gunner and tell him to meet up at the cathedral.

I noticed there was this gate/fence thing in the town square that seems to lead somewhere, but I can't open it. I also went back to the start where there's a crashed helicopter, or some sort of crashed plane. I looked on the radar and it seems something is behind there, but I can't get through anywhere.

Can someone help me out?

Thanks -