Gamma has a simple solution for that.
Imagine you load a static model, for example a rock, and you want to have a corresponding tri-mesh, you just do that :
Graphic3D rock = Model3DLoader.load("data/ARockModel.obj");
TriMeshGeom tmg = new TriMeshGeom(universe);
This provide a high level of abstraction : you don't mind if it's a OBJ, MD2, MD3, 3DS, Cal3D model.
If you have a moving animated object ( for example, a character ) that you want to represent physically with a box, you'll do that :
Graphic3D character = Model3DLoader.load(universe, "data/MyCharacter.md2");
BoxGeom bg = new BoxGeom(universe);
PhysicalBehavior pB = new PhysicalBehavior(universe, bg);
( However, I don't know if you had another project outside Gamma... )