I'm not sold on buying what's marketed as specific gravel bike. There's no rules on how all gravel bikes should be but rather about what suits the rider.
Converted a few older ATB and hybrids for others and myself.
In regards to road vintage, my '72 LeChampion with 700x32c, revised gearing, 'softer' bar and cushy wrap plus Brooks saddle covers gravel trails quite well. Really enjoyable. Funny too as I've ridden gravel trails with a few others while they're on modern 29er atb. They can't get over how swift and lightweight it is. Delightfully deceiving.
To feel real retro, I also enjoy a late 1960s Bottechia with 700x32c Dugast tubular knobbies. Single ring and 5 speed cog.
For all arounds, a 90s era 26in wheel atb works but I much prefer 700c sized.
Older ATB-
I have a parts bin misc. pile of a build 1993 Stumpjumper that fits the ticket as all-rounder and loaner. Tange Prestige with enough braze-ons. Wide Midge On-one bars are my fave. Dual control and added interupter levers. Loaned it a few times and usually difficult for me to get it back! Lol.
Hybrid-
Best of the bunch had surprised me. I acquired a '98 or '99 Cannondale Silk Path hybrid after a neighbor gave up cycling. After a long overdue servicing it now looks new. Acquired a shorter stem and then swapped to road drop bars and dual control shifters, new Conti Speeds 700x42c, Brooks C17. Mid level shift group, overall a fairly heavy bike but it surpasses all of my road bikes in comfort. The Headshock is outstanding with lockout function.