Framing a fireplace is commonly done when using gas or electric fireplaces. You have many choices in placing this type of fireplace and how you want it to look. These factors all make a difference for the finish materials being used around the gas "fireplace" unit its self.
Build the front wall with the R/O (rough opening) of the fireplace just like a standard window. There should be a header above the fireplace R/O to transfer the load from above around the opening. The height that the fireplace R/O starts depends on how it will be finished. Example: if using 12" tile going around the R/O or face of the fireplace unit, the height should be 12", 24" or 36" off the floor so the finish work fits nicely.
After the basic frame/box is built for the fireplace, build a floor and ceiling inside the walls where the unit sits. The floor can be framed out of two 2x4 knee walls with 3/4" OSB covering the floor. The ceiling is done the same way.