Gross was a prejudging way to call obese or overweight people. Gross is from a latin word, more specifically “grossus“, it’s meaning can be thick or large. In the 14th century that word spawned another word which was grocer, which meant a merchant who sold tons of items. See the connection between that grocer word and grocery? Yes they have a history together. To a man studying in finance gross means “without deductions”. To “Gross Out” in the emotion sense means being disgusted by something really over the top, the expression came to be in our standard language around 1960s.
There is an insect in Africa which is called Ptyelus Grossus, it is suppose to be one of the most disgusting things to look at. The images might not horrify you but seeing them in real will turn your stomach upside down.