Change gizmo / pivot position manually?

I can't figure out how to move the gizmo. I imported an .obj with a bunch of models, and I'm only using one of them. 

Currently it's positioned quite a bit to the left of the 3d model, it's positioned in the center of the original obj, but with the other objects gone it's gone off-side. 

Update: my bad. the original additional objects were still in the group. Removed them, set the pivot to "centered", and it's useable. I'd still like to move the pivot manually, but this'll work for now.