Hi Robin!
First, "oats meal" should be "oatmeal". To help remember, try searching for images of oatmeal boxes can help you find how it is used in American/British products.
As many people said: we don't use the word "gruel" anymore.
"Gruel" has a negative connotation for being poor (meaning: not nutritious) food that has no flavor, no substance (because it is mostly water). It reminds people of poverty, starvation, and sadness, so if you said, "His food tasted like gruel!", you would be insulting someone's food.
"Gruel of rice" would be unusual to hear. It's like saying, "Like gruel, except instead of oatmeal, there is rice". People would understand you, but they may not be clear on what exactly it is. It might be better to just show a picture -- sometimes there isn't a good translation!
"eat some gruel as breakfast?" -> Again, because the word is archaic, you will not hear this.
"drink or eat gruel" -> Eat. Gruel, hot cereals (in general), and soups (in general) are considered "food" (and not "drinks"), so we "eat" them.
A good rule of thumb is whether the thing is in a cup/glass/mug: if yes - we probably drink it.
If it is on plate/ in a bowl/ etc. then we probably eat it.
I can't think of any counter-examples right now, so it is a very safe rule.