If I understand
correctly, you have already installed a free-standing gas fireplace unit. You
are now interested in giving this unit the look of a built-in fireplace. Since
the two different types of units are designed differently, it might be
difficult to enclose the free-standing fireplace and give it a properly
finished transition between the wall/surround and the fireplace.
Assuming that you feel you can make this transition look ok, I would suggest
that you build a box that extends completely between floor and ceiling. This
could be smaller above the fireplace so that you can create a deeper mantle top
without extending into the room too much. You will need to make sure that you
follow all of the code and safety requirements for the particular unit you are
working with. Units that are designed to be enclosed allow for appropriate
spacing and airflow to keep too hot surfaces from combustibles. Be sure that
you do not get any construction too close to the unit.
The idea of a built-in fireplace is that it has the appearance of a traditional
brick, wood-burning fireplace. In order to complete the illusion, you need to
have a visual reference to a chimney. If the unit is not recessed into the
wall, there should be structure above the fireplace.
Once the gas unit is enclosed, you can either purchase a manufactured fireplace
surround or build one of your own design. You can simply surround the fireplace
with tile or stone, with or without a wood trim on the outer edge. The mantle
itself can be attached to the wall and extend into the room, or sit on top of
the stepped back box you build to enclose the unit.