Porch roof support question
Gravel will not do you any justice. It is just a cheap way to go deep in ground to get to good bearing soil if it needs to go really deep. Pole barns, for example, are built with 6X6s set in the concrete 4' deep, for a porch roof if it is small enough it won't need 6X6s but 4X4s will suffice if the roof is not too high.