The phrase "the nitty-gritty" refers to the essential details or the core of a matter. To express this idea, there are various synonyms available such as the crux, essence, fundamentals, meat and potatoes, substance, gist, heart, soul, essence, and marrow. Each of these synonyms can be used interchangeably to convey the idea of the most important or fundamental part of something. Using synonyms is an excellent way to add variety to your language and keep your writing fresh and engaging. Therefore, it is always good to expand your vocabulary and learn new words that you can use in place of commonly used phrases.