Do you know ghouls from goblins and ghosts?
1550s, "pleasing to the mind," also "full of gratitude," from obsolete adj. grate "agreeable, thankful," from Latin gratus "pleasing" (see grace). "A most unusual formation" [Weekley]. Hard to think of another case where English uses -ful to make an adjective from an adjective. Related: Gratefully; gratefulness.