hand dig your hole being sure that you are below the frost line. Sorry but my last post assumed you were using horizontal vinyl siding..
anyways hand dig you're hole, place an inch or two of 3/4" clear stone at the bottom of the hole, insert your post. mark where the gas line is on the post and knotch it out, using a hand saw (or skill saw with depth set) and a good sharp chisel.
place post back in place and use post set mix.
Be sure the post is good and plumb. brace and let set.
ask you're builder if he has any PT scraps of fence board... the reason for this is in case the post of off the house at the top of the post even if it is plumb. you will use this scrap as blocking between the house and post. drill and over sized hole (as you do not have 7" screws) and counter sink 3-1/2" screws into your post and blocking into your walls.
Here is an example on my own house. I have decorative block foundation, and a gas line..
this kept everything plumb and sturdy. mind you this is not a gate.. not that it would matter if you did the same using you're 6x6... just be sure to countersink.
G