I have a stall shower that does not have a ptrap under it. Is there something I can put in the 2 inch pipe going down to stop gas smell from coming up

Hi Nbaker

Yes, alternative to a ptrap is shown here... called a PRO SET TRAP GUARD:

https://www.youtube.com/watch?v=IS5pzsxz0oU

This would probably not pass a plumbing inspection in your town for a shower, but it will definitely stop sewer gasses from escaping from the drain.

Good luck!

Mark