What does the expression "in traffic stop gone wrong" mean in the title of the above video?
It isn't an expression. This is typical journalistic language, where the preposition
in heads a phrase that 'reframes' the event in alternative language. (There's an identical example in another current thread today about Elon Musk's rocket.) The noun phrase 'traffic stop' (missing an indefinite article, in typical headline style) refers to when police officers stop someone in their car, and 'gone wrong' is a reduced participle clause, expandable as '[which has] gone wrong', and referring to the less than ideal outcome of the situation.