Some of those new gravel bikes have really slack head tubes. I become suspicious of any road bike with a trail figure exceeding 60 mm. Your Gunnar is at the short side of the spectrum, so I can see why you find it twitchy.
But off the top of my head, I'd steer you toward any of the standard "endurance" bikes--Trek Domane, Specialized Roubais, Cannondale Synapse,or Giant Defy. Of the bunch that I've ridden, I'd say the Defy handled the most like a normal bike. And if the current Scott Addict series (not the Addict RC) handles anything like the second generation CR1, that would be my choice. For me, the CR1 really hit that "disappears underneath you" sweet spot. As for size, use your current reach or top tube length as a guide. With Trek you might want a 56. With the brands that run larger, a 54. I'm betting a M/L Defy would be too big.