Thanks guys. The horizontal slats are going to be spaced about 3/4", but I think, if I inset them in the gate frame, I should be fine. I wasn't sure if I would need to add any steel to support them.
Dave, I've found a pretty good price on PT lumber. How much problem do you think I'll have with warping and twisting if I use the PT for the horizontal slats? I plan to put the cup toward the posts.