Taking into consideration the footing type/trail conditions, I might gait down a gradual slope.
If he is balancing himself, and not getting rushy, and paying attention to his feet, and the overall terrain is just kind of rolling, I don't have a problem with maintaining an easy, consistent forward-moving gait rather than bringing him back to a dog-walk at the top of every hill before we descend.
To clarify - this is not something I "let" my horse do - just as I don't "let" him take off running up hills, either.
He has to be balanced, keeping his weight back and thinking about where he is and not where he wants to be, not heavy on the forehand or rushy.
If in doubt, slow down and walk it. Especially on unfamiliar trail.
There is no shame in that. . .and you and your horse will stand a better chance of living to trail ride another day.
"Parelli horsemanship is just like painting by the numbers. You need absolutely no skill. You just put this color here and this color there, and when you're done, you have ... a mess no one wants." mp