Seems like wheat has the highest gluten and modern wheat has been bred to have much higher gluten levels than historic wheat.
Barley and rye also are gluten-laden.
Seems that oats have some but not much especially compared to wheat. Also rice and corn not much although glutinous rice would not fit this.
So oatmeal porridge or muesli should be ok for breakfast if trying to lose weight but not toast or pancakes. Not bread pizza scones cakes… – all the stuff made with wheat.