When it comes to backfill, the best material is a combination of gravel and native soil. By combination, I don’t mean mixture. But, the retaining wall must be backfilled to a thickness of at least 15 cm by gravel of nominal size not exceeding 10 mm. Then behind that, a layer of native soil as much as the need be. Also, if the land above the backfill is to be used for plant or grass growth, then the native soil layer of not more than 10 cm thickness must be placed vertically above the gravel.
Now, gravel won’t retain water. So, actually, it won’t increase active earth pressure as compared to clay or silt, which could hold water. Also, even if there is a larger design pressure with gravel, still overdesigning is the wiser choice because if not done, the wall or footing may corrode due to water and would have to be rebuilt, creating extra cost.