Something that's ghastly isn't just gross. It's shockingly, horrifyingly unpleasant — so gruesome and grisly that it makes you want to puke.
An adjective used to describe that which inspires visions of death and feelings of revulsion, ghastly originated circa 1300. It is very close in meaning to the similarly spelled ghostly. In fact, the word in Old English was originally spelled gastli. But over the years, people confused gastli with ghost, began to adopt the gh- spelling. But don't you forget the "h" - that would be a ghastly spelling error on your part.