For a long ride on a gravel bike, what is the best frame material?
When selecting a frame / bike the most important factor is that you have decided for yourself what you like in ride feel for your application. Test ride several bikes and develop your knowledge of what feels good to you / fits your riding style and then buy the bike that works best for you. You may want to rent different bikes so you can test them in the environment of your choice.
Most good bike shops will let you test ride bikes. Even though it's only in the parking lot it's better than nothing.
Assuming a reasonable level of material quality, frame design and construction can have a greater effect on ride quality than whether it's made of aluminum, titanium, steel, or carbon fiber. Selecting "the right" gravel frame is not as simple as selecting "the right" material.
By adding material here, removing material there, changing the shape of the tubing, changing the frame angles - and many other techniques a good frame designer will adjust the characteristics of the material to achieve a design goal.
Design goals include (but are not limited to) comfort, or stiffness, or stability, nimbleness, or some combination. Your mission is to find a frame designed to match your needs.
Just one example of this is an old Cannondale SR400 vs. an old Raleigh Technium. Both frames are made of aluminum and both are road bikes The two bikes have very different ride characteristics because they are constructed in very different ways.