Be aware that gravel will take a lot longer to clean then sand. Debris gets embedded in gravel whereas with sand, it tends to lay on top which both makes it easy to see, and much easier to remove rather than having to embed the gravel vac into your gravel and wait until the water clears.
I don't know why people seem to have problems cleaning sand, it is extremely easy, you've just got to be careful that you don't put your gravel vac too close to it or you will suck it up. Because the debris lays on top of the sand, there is no need to embed the gravel vac, the suction will lift the debris of the top.