Scouting 2 Rax & early gas in TvT on a 2p map - what are the possibilities?

First thing to check would be the attachments on his barracks. This will give you an idea of his unit composition. If you do not see a tech lab then the first assumption would be your opponent is going to hit you with reapers very soon. Secondly you will want to throw down a bunker at your natural, and rally your marines into your main worker line.

Two marines handle 1 reaper, and in a larger scale you will not need as many marines. To better handle the scenario would be to get a hellion out asap. Your opponent has a quick gas which means they have the ability to tech faster, in the long run this could lead into a banshee harass. You want to get the factory down soon anyway to open up your tech.

Also you can throw down a second barracks yourself. The time that it will take your opponent to traverse the map is extra time for you to increase your army supply. If we both throw down our first barracks at the same time, I throw down my second barracks 40 seconds before your second barracks, and it takes me 40 seconds to get to your base. Our army values should be equivalent when I reach your base.

You should never need to wall outside of a TvZ matchup, if you do you are giving your opponent free buildings to pick off should they have the superior army. Personally I would pull the scv out of the base and have it patrol in the center of the map, or take a watchtower. Your first card was the expansion which means that your opponent has to either react by building his economy (expansion or orbital) or by crippling yours. You scouted he was not building up his economy so the next logical move would be to assume he plans an early attack, and your move should be scouting what it is / when it crosses the map.