Bitter gall

I was reading Proverbs 5 and saw the phrase "bitter as gall," which made me wonder what the heck "gall" was. It seems that it's bile, but initial definitions, at least in my copy of Webster's Encyclopedic Unabridged Dictionary of the English Language, describe "gall" as "something bitter or severe" or "bitterness of spirit." Which is odd, since the proverb compares bitterness with gall. So what's originally written as a noun becomes an adjective, using the comparison to become the definition itself.