The word "gullion" is quite rare and not commonly used in everyday language. In fact, it is difficult to find exact synonyms for it. However, some possible substitutes for "gullion" include "slag," "dross," "scoria," and "cinder." These words all basically refer to waste material or residue that is left over after some sort of industrial or metallurgical process. While these terms may be used interchangeably in certain contexts, it is important to note that they may have slightly different connotations or origins. Regardless, if you are looking for a synonym for "gullion," any of these words should work just fine.