I believe panaracer makes a 26 inch version of the Gravelking tire. Schwalbe makes the marathon touring tire in a 26 inch version as well. I think the Ritchey Tom Slick and Tioga City Slicker, Maxxis Hookworm, Serfas Drifter, Continental Top Contact, Kenda Kwest are all relatively skinny 26 inch tires that could do gravel.
FWIW a modern gravel bike is a bit different than an old fully rigid 26'er with slicks or small knobs, but if someone doesn't prize top speed / efficiency then they certainly make perfectly adequate gravel grinders.