I have built a couple of lugged gravel bikes. Unterhausen is correct that you will likely need to bend the chainstay ports outward to accommodate wider tires. If you build the fork yourself there a few options of wide crowns on the market, Pacenti comes to mind. You could use cantilever brakes on a gravel bike but then your limited to one wheel size. I'm not typically in the disc camp for road bikes but on a gravel bike it makes sense because sometimes a 650x48 is the right tire and sometimes 700x38 is the way to go. As you mentioned gravel bikes are a bit more relaxed than a road frame, be aware of wheel flop when designing the front end which becomes more a reality when you slacken the head angle and kick out the fork rake. Also, compared to a CX bike the bb drop can come way down. Lastly Rene Herse sells their BB shell with matching bent chainstays that clear a lot of space in the back, if you don't want to do a lot of manipulation it's a nice option, though almost 2x the cost of any other bb shell/chainstay combo. Here's a stainless steel lugged gravel bike I build last year.
