Use a physics constraint and tie the object to something thats invisable (maybe a func wall, clear unbreakable glass, im not sure if the nodraw texture would work). Place some sort of sprite to make it look like energy from the gravity gun above, below or whatever. maybe you could make your invisable stand rotate very slowly to give the ilusion of suspension.
If your wanting to do this with some entity like a gun or a monster I have no idea.
If it sounds like I have no idea what im talking about, I dont. but thats the first thing I would try.