You have to ride a bike made out of steel I-beam at 300 pounds.
Actually you probably can ride carbon fiber as I ride my carbon fiber road bike and I weigh 365 pounds.
Gravel shouldn't be too much more abusive to the bike than chipseal or other road surfaces.
Aluminum is a good choice as would be any of the other frame materials.
Your wheels would be the week spot, and those are easily upgraded.