Any tips on how to cook gruel?
Rice gruel or congee is made with whole grain rice like Megha said. Chinese congee has already cooked rice added to a meat broth and cooked long enough so the grains start breaking apart and becomes thickened.

If you attempt to cook glutinous (sticky) rice flour with water, you're making something more like mochi, although the traditional way is to pound cooked rice into mochi. Most home recipes for mochi use the microwave to cook the rice flour and water. If you want rice gruel, I'd advise you to use cooked rice. It's how I do it.