Mouse coming through gap around gas pipe
This is a common issue that is often ignored or dealt with in shoddy ways. First, yes, set a mouse trap and get rid of the mouse. Then deal with the hole.

You may find an escutcheon that will fit around the gas pipe. Gas pipes may be many different sizes. To keep the mouse from pushing it up you could use a pipe clamp above it and/or glue to secure the escutcheon.





If you can't find an escutcheon, then you can make something to fit over the pipe.

Get a piece of plywood, maybe 6 inches square and cut it in half. Drill screw holes in the corners of the two pieces. Screw both halves down (butted together) onto another piece of plywood. In the center (on the seam) cut a hole the diameter of the pipe. Unscrew both halves an place the two halves on either side of the pipe (covering the mouse hole) and screw it down.



tip: the reason we cut the board in half first, is because most circular saws will remove 1/8" as they cut. If you only have say, a 1/2 inch pipe, then after you cut the hole in half you will have a football shaped hole instead of a circular hole... and the mouse will probably start gnawing at the gap.