No, it's not from vent gleet, unless you noticed at some point that she's been nibbling at rotten fruit or veggies or drinking from a scummy puddle.
Large hens can have an issue with "lazy pooping". Often, they accumulate a fat "shelf" under their vent that collects poop that fails to shoot out and clear the vent fluff. I have a few of these "plus-size" girls. They get frequent butt baths.
I back the hen up to a basin of warm water, splash enough to soften the poop, maybe use a little baby shampoo or body wash, and then rinse. They adore being blow-dried, or just let them air dry on a nice day.