Here are some synonyms for "ghastly" as an adjective:
* Terrifying
* Horrifying
* Macabre
* Grim
* Pale
* Sickly
* Dreadful
* Appalling
* Spectacular (in a negative way)
* Unpleasant
For example:
* "The house was filled with a ghastly silence."
* "He had a ghastly look on his face."
* "The news was ghastly, and the world was in shock."