It depends on the speed you want to run at, but there are plenty of choices.
I'm a fan of the Maxxis Hookworm. I've used them for a decade and have abused them in ways that most tires wouldn't survive. They have a thick DH casing with heavy sidewalls and a tread that extends far onto the sidewall.
CST Cyclops used to be the same tire as the hookworm. CST is the parent company of Maxxis, and for a while, they sold the Cyclops along side the Hookworm, but for 1/3 the price and a different visual tread pattern. a few years ago they changed things up, and the Cyclops now has thinner sidewalls, but weighs much less. It's still as thick through the tread, and is a very stout tire.
Both are balloon tires, listed as 2.5" and 2.4" respectively. they would do well on loose gravel.
Schwalbe make the Big Apple, that everyone's heard of, but it's not their best tire. They also make the Marathon series. the best being the Marathon Plus HS 440. It's ES50 rated, which is a European Speed rating for electric bikes meaning it's certified up to 50kph. Many others are only ES25 rated. It also features a protective strip in the tread to guard against punctures. It's a 2" tire, so not a balloon tire, and better suited to packed gravel.