You're right, they mean the same thing. "Nitty-gritty" is African-American in origin and it entered the main language in the 1960s. Wentworth and Flexner's 1967 "Dictionary of American Slang" has it in the supplement, and their definition is "the basics or essentials of any situation, predicament, action, etc. esp. the hard, unvarnished facts, 'brass tacks,' or harsh realities." The American Heritage dictionary calls it "informal" and "origin unknown." They list "brass tacks" in the main section, with a 1914 example of use, so they consider it to be part of American slang, but they don't give any origin.

Hi Artem, "Brass tacks" is also British but actually in my experience a little old-fashioned now. I had always thought it was Cockney rhyming slang for "facts" but don't quote me. I don't know about the usage of either in the USA. The meaning of both expressions is the same - the crux of the matter, the fine detail, the nuts and bolts. get down to the nitty-gritty get down to brass tacks

Both are very American in usage. Brass tacks is rather old-fashioned these days. 'Nitty Gritty' goes back in the U.S. to at least 1966 with the founding of the American band "The Nitty Gritty Dirt Band"