Drum roll....
It depends 
It really does X_x
It's mostly personal preference. Deeper rims are generally heavier, more stiff and more aero. Some people like more compliant, shallow wheels for gravel and stick with shallower rims. If your riding is on more flat and non-technical terrain, maybe something more aero and a tiny bit heavier is good. If you're doing a lot of climbing/descending, especially on technical or bumpy terrain, maybe you want something shallower + lighter for the climbs and with a little more compliance on the descents.
Inner width is probably more critical if you are trying to aero-optimize a road bike. For gravel, a wider rim will stretch your tire a little wider and probably be a little more aero. Less difference between your tire width and outer rim width = more aero. For gravel, anything from 21-27mm is probably fine. If you care about being more aero or making your tires wider, maybe aim for the higher end. If not, it doesn't really matter.
I like to squeeze as wide a tire onto my bike as possible. I had a set of 2.2" Race Kings that I moved them from a wheelset with a 22mm internal width to a wheelset with a 25mm internal width and they started rubbing on the fork because the wider rims stretched them out more... so there is that to consider. If you're not maxing out your tire clearance it shouldn't be an issue though.
I don't have a lot to say about spokes. If you live somewhere by the ocean or where they salt the roads in the winter, maybe stay away from aluminum spoke nipples because they'll corrode. Sapim spokes are sort of "industry standard," they're good. I hear Pillar spokes are fine too. IMO titanium spokes are kind of a gimmick and carbon spokes have no business on off-road wheels. Especially for gravel, I don't think you can really go wrong by just sticking to whatever steel spokes.
Aluminum wheelsets are often 1800g+. Moving to any <1500g carbon wheelset will probably make a noticeable difference.
