Yep, was right. It will be reopened to pedestrians and cyclists.
The Mill Road Bridge has been rehabilitated, but subsequently closed to traffic. It it is connected to a bike path that leads into downtown Gettysburg, and from there, a rail-to-trail that currently leads eastward. A trail extension to the west, on the old PRR, will provide connectivity to other systems.
But until that trail is completed, a bike route is signed along Bridge Street via a two-lane route and on some other back roads. Mill Road would be a reroute of that.
For some reason, they put up barricades so that it takes some effort to climb over and get onto the bridge. It's open to pedestrians (and I presume cyclists), but it's just easier to take Bridge Street.