You want about 11kg/ 25lb of gravel for every inch depth of substrate in that size of tank. So 2 inch of substrate would be 22kg/ 50lb.
1 betta in a 20g tank? Just do a fish in cycle. That volume of water with just 1 small fish in it should never see any elevated water parameters. Just monitor water parameters, you will probably have to change water weekly for a couple of months, and after that you should get away with monthly water changes. All that will happen if you fishless cycle is the bacteria you grow while cycling will die off once you add your fish because your lone fish will have such low bioload it wont support all the bacteria you put all the effort into growing.
We get a lot of traffic on this site with people not being able to get tanks cycled. By far more people have issues with fishless cycles. They dont understand how to do it, or expect it run to a timetable. Save yourself the grief of looking at an empty tank for several months, buy a fish and do a fish in cycle.
