If it's anything like our new build you won't be able to do anything with the faux chimney - I understand ours is solid whilst the top 4 feet or so is fibreglass. You can't tell by looking at it - I only realised when I saw about 5 'chimney stacks' ready to be installed on newer houses. Guess yours might be different.
We got a marble fireplace installed but haven't fitted a gas fire or alternative. It's purely decorative. The central heating keeps the place warm but I do miss a real fire at times.
ATB
Jon