If you google the fireplace model number and "installation instructions" you will see that there is a 1/2" thick non-combustible surround that is shipped with the unit that is quite high on top and 1 3/4" wide at the sides and about 10" at the bottom (if the unit is not installed at the floor level with a hearth). This material cannot have drywall or another combustible material over it.

It appears that you will need to apply a non-combustible plaster material over the factory supplied non-combustible board or cover it or the entire wall with a similar con-combustible material (cement backer board, tile, granite surround?) and finish it.

It is the joint between the unit and the non-combustible materials that is covered by the trim.

@Renovator8 - thanks for your comments.

If we painted the non-combustible plaster material that's over the non-combustible board..would it not look smooth? would there be a substantial difference in color or consistency?

A plasterer could skim coat a veneer plaster over durock.
Casey

We have just purchased the same fireplace. We wanted the same clean look that is in the picture above, but our installer is telling us that the access panel at the bottom of the fireplace has to be removable. It can be covered with some sort of noncombustible material that can be glued to it, but there will be seams at each end because the panel still needs to be removable. Did you cover the access panel at the bottom of the fireplace? How did you ultimately handle the installation? Did you use the cement backerboard and paint?

Caulk the crack with a flexible paintable sealant (not silicone), wipe off excess till smooth with a wet cloth, then paint.