Should I use wood or metal for an 8" gate post?
Depending on where you live may change the answer slightly.  For instance, I live in the Seattle area, and an 8" cedar post is used frequently.  It's usually quite wet up here, so metal posts often rust, especially if they're not cared for.  The other plus for a wood post is that it looks better (in my opinion).  Downside of a wood post is potential warping.  I've seen some 6x6 pressure treated posts returned at Home Depot that were literally twisted almost 60 degrees.

If you live somewhere a little dryer I would think a galvanized steel post would work well and last longer.