Based on the photo it's likely they framed all that out, so you'll need to add drywall and floor trim for the sections of wall that you'll regain. You'll want to cap and seal the chimney, ideally at the top where it vents to atmosphere so you don't have any cold/hot most air condensing inside. Definitely cut off the gas. If it's electric, then I'd use that circuit to add an outlet in your new wall section.