Common frame materials include carbon fiber, aluminum, steel, and titanium. They all have a different ride feel and give a different character to a bike but also, it's not as important as it seems. Where you really start to notice frame material differences is when manufacturers are able to use the inherent properties of a material to tune ride feel.
Carbon in particular benefits from an ability to tune ride feel. Because carbon fiber frames use sheets of carbon fiber laid into a mold in a particular order there's a lot of adjustability. A manufacturer can turn sheets in different directions, or use different types of carbon fiber, in particular areas. That lets them build compliance into a frame in very precise ways. Skilled builders can also use metal to accomplish some of the same goals. At the lower-end price points, there's a lot less of that happening in the frame design.
For a gravel bike under $1,000 it's most likely you are going to find aluminum is the material of choice. Aluminum is easy to work with, stiff, and light. Outside of aluminum, the only other material you might find is steel. A steel frame is repairable if something happens and the ride isn't as stiff. The disadvantage is steel is heavier and adds some extra weight to the bike.