Is the part you want to fill that little strip? Is the greyish thing at the top right of the photo a mat in front of your front door? I am assuming you are not talking about that ~2x6 foot planter area adjacent to the concrete, just the 4 inch wide strip directly in front of the gas meter?

If just the strip, pull out what is there - a claw hammer and hand trowel are probably all you need. Did it the same depth as the remaining concrete, put a board to form an edge at the end and mix and pour some concrete mix. Like one other poster said wrap the vertical gas line to separate it from concrete, even a few wraps of cardboard would do, it will rot away if you can't pull it out. I would put a 4 inch wide line of painters tape on each side of the trench to make it easier to do a clean pour.