The type of gravel matters less than the chunk size. Smaller is better for plant roots.
There is the reverse flow ugf that some have done with plants. It sucks the dirt down the tube and lets it accumulate under the substrate, breaking down slowly as plant food.
But I personally prefer sand or soil with plants. If you are planting densely enough, and keep your lights on an automatic timer, the plants can replace the UGF in a moderately stocked tank. They will gladly eat up all the ammonia and some of the nitrates. The surface area of the plants also provides surface area for bacteria just like the ugf does. Use the power head or air pump you were running the UGF with to provide general current in the tank.
I have run my 75 gallon for several years massively underfiltered, with just plants as my main biological filtration. The filtration I do have is just to pick up the sand dust or mulm in the water when I stir up the sand for some reason.
