Slicks work well enough on the occasional gravel road. You don't give any idea what type bike this is for. But for up to about 32mm wide tire, I'd recommend Continental GP5000 in one of their half dozen or so different types. I used their tubed version for a long many years. Even the 25mm was adequate for short stretches of gravel.

You don't need knobs at all. Especially if it's only occasional. If you are worried about flats and such, then go tubeless. If blazing speed isn't the thing you ride for, then maybe even run flat inserts might something to try.