This depends largely on the configuration of your fireplace insert. Some, like the one in this illustration, acts like a heat exchanger. Room temperature air enters the box around the firebox, heat is transferred to this air, and it exits -- significantly warmer, but not hot enough to be dangerous. Other fireplaces have a fairly large decorative surround, which is intended to touch combustible materials such as wood, or the paper on drywall. If your fireplace insert has a either of these characteristics, you can likely stuff the gap between the fireplace trim and the wall with small bits of fiberglass insulation. It is noncombustible. This, of course, is dependent upon the joint between the fireplace insert and the wall. If it is an overlapped joint, the insulation will not be visible. If it is a butt joint, you may require a trim piece. This is where gets tricky, because a piece of wood trim would be the easiest to install, however the size of the surround, and the temperature, where the trim would be installed, dictatewhether you can use anything combustible. Metal trim pieces are of course harder to come by, and more difficult to work with.