David, I did this last year and learned quite a bit. I put a carpenters
square alongside house and used that to direct a taught stringline. All
posts must hit that line. As for gate, I put a 5' on one side and 4' on
the other side of house. I did not put aluminum insert into "hinge"
post nor fill it up with concrete. That 1/4" thick post is not holding
the gate up enough. I need to remove the hinge screws and get the
aluminum insert down into post to get the strong spine established.
When you install posts, I measured forever and found it easiest to dig
(power augers are too much work) each post hole around 12" diameter.
Get the $40 post hole digger from Depot and the $30 big thick heavy
steel "shale" bar with points on each end. They are in same aisle as
post hole digger. The digging went very quick in my New Jersey
clay/shale soil. When I installed gate posts, I tool (2) 2x4's and
clamped them horizontal to ground across the gate posts. One at ground
level, the other at top of posts (now in hole). That sets the gap
between gate posts at exact dimensions. This is a very critical step.
When all posts are in position, pour cement into each hole. I filled
hole till about 8" below ground level. Also, I wanted my truck to get
into back yard, so I got a set of "clips" that screw into posts. I
"just" slide the fence section in or out and it takes 2 seconds to open
fence to get my truck thru.

The manuf. typically suggest you drill holes in the lower section of
posts and put a piece of rebar thru. That would be embedded in the
cement and otherwise lock the post from creeping up. This is great, if
you will NEVER need to remove a post. I did not do this, and its a good
thing because I need to remove a fence section due to some remodeling.

Good luck, I don't regret it but now need a power washer to keep it
clean. More tools !

Tom