I think the closer to low-20s in gear inches the better, under 20 even nicer for heavy loads. But most gravel bikes i see are mid 20s.
Lots of $2,000 gravel bikes with 700c wheels. Have a 650b set built for around $380-$450 (Hunt has a set for $409), sell the original 700 wheels. You don't need lots of 650b wheel options to choose from, you just need one good set.
How much weight do you plan to haul, and what kind of terrain? I did a short lightweight bike packing on my 48/32 x 11/34 Norco Search XR last summer, in the Sierra Natl Forest of California, and it worked great on the $425 Kinlin rims/Bitex hub 650b wheels i had built, now running Schwalbe 2.1 Thunder Burts. I see no reason to pull out the original 700c x 40 wheels i have stashed in my closet unless, maybe, to sell.
The latest Norco Search XR S2 (latest version of my 2018) is $2,000. It's only 10-speed, but has GRX 46/30 crank, and 11/36 cassette, and GRX hydraulic brakes. Double mounts on the fork blades, rear rack mounts, inner triangle bottle mounts and one beneath the downtube, and thru-axle front and back. Comes with 700c x 42, but maybe a shop will swap a 650b set for you at purchase. This is just one example of what's out there. SO many options these days:
https://www.norco.com/bikes/2020/dir.../search-xr-s2/
Build up some 650b wheels, add a RedShift StopShock stem, 30 chainring, 36 large cassette cog, you would have a great bike packing bike.
(i'm mentioning the Norco Search model mainly because i have one, and just saw the latest 2020 model in my shop the other day, and the blue really caught my eye, it's a nice looking bike that can take both 700c and 650b.)
eric/fresno, ca.