The best gravel bikes under $1,000 / £1,000 will almost invariably include an aluminium (also known as alloy) frameset.
Aluminium bike frames are comparatively cheap to manufacture and will give you a reliable ride that should be comfortable. While most of the best gravel bikes are made of carbon, with a few made of titanium or steel, aluminium is a perfectly good material if you're looking for a budget gravel bike.
It's worth looking for a carbon fork rather than a metal one though, as it should help to take some of the surface buzz out of your ride.