Nice work.
Those RLT/RDO bikes are sweet. I'd have one if I had the money and the opportunity for a good test ride! I hear I might find one available for test rides in Northern Georgia...
Fixed gear is a good workout.
riding with road bikes, I would rather be on my gravel bike than fixed gear - or what usually happens is that I will drop down to a slower ride and get a bigger workout riding fixed.
I'm doing 42x16 for a typical "spiritied" road ride (average 20mph, maybe 25 max). For a 49x15, I'm doing 25mph with peaks of low thirties. I've thought of doing something like that on the ride I referneced above, but I would get dropped on the hills - come to think of it I would be as likely to get dropped going down hill as up hill**. ;-)
Back on topic - a gravel bike, at its basics is a road bike that can take 40mm+ tires. There is a huge variety of this "new" segment.
Build it like a road bike, and it will be as fast as a road bike
Build it like a backpacking bike, and it will be as fast as a touring bike
Build it like a monster cross, and it will be as fast as a mountain bike.
Put slower tires on it, and it will be slower, put the same tires/wheels on it and it will be basically as fast on a simliar course.
Like you said, its a great sport! its all good!
**the proper gear on a fixed gear bike is one where going down hill hurts as much as going up hill