I've never been able to get hold of Evaporust when I needed it, so I used a rust converter solution that's
closer to home.
If you're not in any hurry, mix up some of that stuff, leave it in the tank for a few days and agitate periodically. I'm sure the instructions claim it acts faster, but my experience is that the longer you leave it (within reason) the better. If the rust is only surface deep, it'll clean it all out. Any heavier and it'll work, but for piece of mind you'll probably want to go the electrolysis route as well, just in case.
The difference between the chemical and mechanical methods are that the chemical options convert what's already there; in that sense, they're non-reductive (not the right word but you get what I mean). With mechanical methods like gravel, you're taking stuff off. Personally, I'd go with the conversion routes esp. with the low impact converter solutions because they tend to be very gentle.