Question

"What is the main difference between galls and calluses?

Answer 1

As nouns the difference between galland callus

is that gall is (anatomy|obsolete|uncountable) bile, especially that of an animal; the greenish, profoundly bitter-tasting fluid found in bile ducts and gall bladders, structures associated with the liver or gall can be (countable) a blister or tumor-like growth found on the surface of plants, caused by burrowing of insect larvae into the living tissues, especially that of the common oak gall wasp while callus is a hardened area of the skin (especially on the foot or hand) caused by repeated friction, wear or use. 

As verbs the difference between galland callus

is that gall is to trouble or bother or gallcan be to impregnate with a decoction of gallnuts in dyeing while callus is to form such hardened tissue.