Bad idea to bolt it to the house. That’s asking for water issues and why screw with the siding.
Newmans idea is the best one, and is what we do to cover up steal supports on things like porches where you don’t want to see structural necessities in raw form.
I know they make pressure treated “1x” material. If lowes or depot don’t have it check out one of lumber outfits. We get most of our material from Mosher Lumber in Clearance.
Piloting the holes and using stainless fasteners should allow this to work very well and last a long time. Using some washers and nuts/bolts to hold it to the pipe itself is also a good idea. I wouldn’t hesitate to glue the seams. Titebound III is a good wood glue. Something with “outdoor and waterproofing” on the label should hold up.
If your new posts are more than just a flat top, you could also attach one of the decorative tops to it for a finishing touch.
Time to test out the new table saw and do some treated lumber ripping!
On another note, your not going to hurt a gas line with a shovel unless you already have a problem that’s waiting to peak its ugly head.
EDIT: ^Ceder is also a good choice, usually a bit more expensive than treated however
I have hit them with backhoe/excavator buckets before and have never had a problem, there is no way you and a shovel are going to do any real damage unless you are really trying, or the line is messed up to begin with. I’d just pull out the old post if it was me, we do it all the time and it’s really not too hard to do.