The gross indecency was just being openly gay, or at least open by the standards of the late 1800s. The Marquess of Queensbury wrote him a letter accusing him, Wilde sued for libel, and that's when Wilde was convicted. Wilde was brilliant, but his imprisonment broke him. Rupert Everett made a movie, "The Happy Prince", about Wilde's last days - it's in my netflix queue but I haven't watched the whole thing yet.