A Gustav gun needs a fixed or limited naval mount

Would mounting a spare Gustav gun on a battleship require a fixed centerline installation?

Probably. A fixed or limited-traverse mount is the practical option unless the ship is made large enough to carry an enormous rotating turret.

A ship could carry the enormous gun on the centerline, but an ordinary rotating turret would be difficult to accommodate. A fixed fore-and-aft installation is the most straightforward arrangement. A mounting with only limited traverse offers another possible compromise. Full turret movement would require a much larger ship built around the weapon.