Hoagie...it doesn't make sense. CO is deadly. No one in their right mind would try to sequester CO within the building. You try first to not allow its development, and secondly to ventilate adequately to get rid of it.
Further, a 6" step would do little or nothing to mitigate gas migration, even gases that are heavier than air. They would migrate by differential pressure and air currents. Only positive seals will help against gases...a 6" curb, step or other "obstruction" would do nothing for that.
The National Fire Protection Assoc. (NFPA) has standards for detecting and warning against CO, but no standard for sequestering it.
I wasn't referring to errant stormwater. I was referring to the building code requirement for a garage floor to drain to the outside. On the dwelling side, this is usually mitigated by providing a differential slab elevation or "step".
I sincerely hope you do not incorporate this "sequestering" concept in a design. In my humble opinion, to do so it would constitute professional negligence. If you do, make sure your liability insurance premiums are paid.