Weight matters, but not as much as it seems at first glance. For riding on gravel and unpaved terrain, the difference between 9 and 14 kilograms is only noticeable on steep climbs or when you're carrying the bike yourself - like hauling it over an obstacle or loading it into a car. On flat ground and downhill, you barely notice that extra kilogram or two. What really counts on a gravel bike is frame geometry, tire quality and pressure, not grams of carbon.
The 11-12 kilogram range mentioned above is genuinely the sweet spot. It means the bike is built with decent components - they didn't skimp on hydraulic brakes and drivetrain. But a 9-kilogram bike can be good too - it's just usually either a carbon frame with lightweight parts, or you're overpaying for the brand. At 14 kilograms, they start cutting weight in weird places: steel frame, cheap parts, heavy rims.
So focus more on what's actually in the spec: mechanical or hydraulic, what drivetrain, frame material. And I agree about the seat - a comfortable saddle does more for you than half a kilogram saved. If you're sitting in pain all day with a sore tailbone, no light bike is going to help.