Do I need a plumber for this?

I was wondering what kind of regulator I would need to take my 2 inch gas line coming into my house and drop it down safetly to 1 inch / 3/4" size?

Here's my garage floor drain that I have never looked at, until today. [ATTACH]79264[/ATTACH] I was cleaning the garage and all the sawdust etc. I had some rubber mats over the drain for months and noticed it was dry. So I removed the strainer and decided to really get in there and clear all the debris. There was some pretty caked stuff in there but I got it mostly out with the shopvac. It took a while to notice that the cleanout plug was missing, which is when I stopped. I'm no plumbing expert but felt like I knew how a floor drain works. Why have I not smelled any sewer gases in the garage during the several years I've lived here? Most of the time I haven't covered the drain. I'm guessing the cleanout plug doesn't just evaporate one day? I confirmed that this drain is connected to the main sewer line by flushing a toilet. I can clearly hear water running. My theory is that the sewer gases have a lower resistance path going up toward the main stack so nothing noticeable comes into the garage. In case it helps the diagnosis: the garage is the last area the sewer line runs through before exiting the house. I've never dug it out but there's a big cleanout plug in the floor just before the garage door and I'm pretty sure of this. Where do I find an expanding plug for this? Any other things I should check? I went to the local hardware store and only found a 1 1/2" test plug but this seems to be closer to 1 3/4 since it didn't tighten snugly. It certainly doesn't look like a 2" hole, which was the next size they had.