Late entry to this one and mainly to say that 650B versions of the Thorn Mercury Mk 3 or Mercury 40 will do excellent duty as gravel bikes for certain purposes, and possibly for most purposes, and have the advantage that the frames are Rohloff specific (but maybe cost more than the OP wants to spend).
I've no personal experience of the 40 but the Merc Mk 3 650B can be a very comfortable ride on gravel, with a compliant steel frame and optionally a compliant steel fork (for those happy to use a rim brake up front). It handles 48mm tyres very comfortably and the specs say it can mount 54s.
It's what I'd pick for a mixed-surface multi-day tourer, which I would say since that's what I got it for. It's also superb on the mix of sealed and gravel cycle paths I do day trips on. I run straight handlebars.
But I ride mainly by myself and at a pace that despite my best efforts remains leisurely. Here in Oz anyway, quite a few people seem to participate in a quicker, more competitive form of gravel riding that's usually undertaken in small groups or as part of organised competitions.
Essentially it undertakes the sort of day trip you might do in a group with a road bike, but on gravel roads. A part of the point is to maximise pace.
I suspect a good rider could do pretty well in such a group on a Mercury, but the preferred steed seems to be a a few pounds lighter, rolling on wide 700C tyres and with drop bars that help you maximise speeds (and draft your friends) on descents. Part of the point of having such a bike is that you know you're not giving up anything to your mates on equipment, because theirs is the same. The gear set-up is always derailleur.
IMO once you go to Rohloff you may as well go all the way and get a Mercury. You could still use drop "gravel" bars if you think that's important, and the Rohloff shifter will be no harder to manage on a drop-bar Merc than on anything else with drop 'bars. You could fit an 853 fork, if you could find one in the right offset, which would shave weight and maximise comfort. And for the sake of 600g you could emphasise your non-conformity and run mudguards.
There would be a fair bit of satisfaction in staying with the gravel-bike pack on a Merc set up light, IMO, and I doubt you'd be giving away much.