Corn gnome(s) is a single gnome or a group of gnomes that live in your colon. They take the chewed up corn out of your poop and put it back in your poop; in whole kernels.
Lore has it that they do it for a more festive bowel moving experience.
You had corn on the cob last night, you remember chewing your food well but there is whole kernels of corn in your poop; "it was the corn gnomes."
You haven't ate corn in almost a week but, you have whole kernels of corn in your poop; "it was the corn gnomes". They keep some stashed away, so when you haven't ate corn in a while they have something to do when they get bored.