“The most beautiful words in the English language are 'not guilty'.”
Did he say or write this (or a Russian equivalent)?
If so, what was the Russian original? In what context (why/where/when) was it introduced by Gorky?
The only relevant source in Russian I could find - the book of a modern writer, Albert Tussein, on proza.ru. This is just a collection of quotes and anecdotes and it has this quote, attributing it to Gorky. http://proza.ru/2014/07/05/656
Google doesn't know about other books with this quote (using Tussein's wording), parts or variations of it. Search for "most beautiful", "not guilty" etc. in combination with Gorky's name brings nothing.
So either Gorky never said something like this and Tussein misattributed the quote, Google (and Duckduckgo) do not index the source or the source isn't digitized.