The word "gristliest" can refer to something that is tough, fibrous or sinewy in texture. Some synonyms to describe such toughness might include stringy, chewy, leathery, or even rubbery. Other words that could be related to gristliness might include unyielding, stubborn, or inflexible. When describing meat or food, some additional alternatives could include sinewy, gamy, or gamey - all of which describe toughness or a strong flavor profile. Ultimately, the context and intended meaning of "gristliest" will help to determine the best synonyms to use in any given situation.