Sorry to hi-jack but whats the general sizing advice looking at gravel bikes? I seem to fall between a 56 & 58cm on lots of bikes. Is reach/TT etc as noticeable as on a MTB? Or will 10 or 20mm more on a stem be a good way to adjust a 56cm frame but maintain low standover?
I wouldn’t worry about standover, and bigger frame triangles are easier to shoulder, and unless you spend a disproportionate amount of time stood over your bike and have really stumpy legs it’s not something that should matter anyway.
Have you got a road bike to compare the sizes against? There’s pro’s and con’s of changing stem lengths. If you started at 110mm and went down to 90mm the steering will get quicker (remember long reach and slack head angles both add stability, and short stems bring the steering feel back to something more neutral), conversely a 130mm stem will slow it down slightly.
Most roadies would probably buy the smaller bike and add 10mm to the stem, because no one really (thinks they) wants a bike that’s a handful to ride and fashion/tradition. Most MTB’ers would probably do the opposite, because they (think they) want the long wheelbase, and fashion. Neither is entirely right or wrong but if you’re changing it by 20mm you’ve bought the wrong frame size. Geometry is always a set of compromises, you wouldn’t try and make a LLS bike fit by putting a 20mm longer stem on it, because it’s nicely balanced and stable with a 35mm stem. Similarly ‘road’ bikes have evolved into a different set of numbers which are equally nicely balanced and stable in a different way.
If you’re between sizes, I’d tend to go with the smaller frame, it’s easier to get comfortable on a bike that’s 10mm too short, than to spend all day stretched out a little too far or not be able to reach the drops comfortably. Just beat in mind the seat angle has a big effect on “reach”, a 56cm horizontal top tube on a frame with a 74deg seat angle and a setback seatpost is about 2cm longer in reality than a 56cm top tube on a 72deg seat angle with a more inline post. So it’s worth looking carefully at the bike you want’s geometry, not blindly searching for a frame size. A couple of outliers I can think of are the planet X track bike, the 54cm medium is good for upto 6ft1 because it’s so aggressive, Cube’s sportive bikes on the other hand I think it was upto a 60cm frame (virtual, it has a sloping top tube so it’s actually measuring stack) to get the controls in the same position as my 56cm Cannondale, which when I bought it was more like a 58 from other brands.