Not a lot of input on the gravel grinding but I started out one of my recent mountain bikes as a dual purpose bike with cyclo-cross tires, touring gearing, etc.
What I found out was I actually can ride just as fast with 2.4/2.2" fr/rr fast rolling xc tires on both gravel, pavement and hardpack and also have a very nice rigid trail bike. I found a 2x10 gear setup that works for me (and I had it 1x10 at one time). The only issue is chainline. To make a bike like this you wind up using a poor chainline most of the time i.e. on the trails I'm using 1 through 4, maybe 5. On the road I'm using 7 through 10, and dropping down to 1 for a longer climb. I'm almost never in 5 which is my ideal chainline. When I had the bike setup as a 3x10, I didn't use all the gears, except going down hills on asphalt, but I was cranking along with a much more efficient chainline. With the 2x10 I don't really need the granny ring, but I need to stand to power through steep climbs, so it's more like riding a SS (but not nearly as hard). That's only on trails though. I can make it up any fireroad or paved road in 1x10 mode. I was using a 38T front ring with the 700x41 tires and I think I could have good 1x10 gearing with a 36T front ring with the 29x2.2 rear tire. My 2x10 is 38/26 and I've been using a 36-11 cassette all along. FWIW on my dedicated MTB, I'm using a 30T front ring with a 36-11, and a 29x2.2 in the rear. It's not enough gear for road riding but it's perfect for trails.
Anyway, I have these two bikes:
Green bike is "a little bit country,":
Black bike is "a little bit rock 'n roll.":
They are both the same medium sized Karate Monkey frames.
Green bike does everything for me from pavement to trail. The black bike is a dedicated trail bike only.
Main differences are 100mm front suspension, lower gearing, tires and dropper post on the dedicated trail bike.
It cost me a little over $1200 to build the rigid green bike, but I had a few parts laying around and I've switched a lot of stuff to fine tune. Building a custom bike is pretty easy, and fun, but it costs a little more.