What does this sentence mean? "I'd go full Gitmo on the old biddy" I hear it in a movie
Gitmo: Guantanamo Bay, the place where the US military imprisoned terror suspects to extract information - notably in the news using "enhanced iterrogation tactics" like waterboarding
Old Biddy: colloquial americanism for an elderly lady.
In American english slang, if you are going to "go (something) on someone" (and often the phrase is "go (something) on your ass"), it is implied that I will be acting in a manner consistent with that (something) to you.
I might tell my kid if they misbehave that I will "go old-testament on your ass", implying that their punishment will be harsh and inflexible as were biblical punishments in that book.
So one can deduce that the speaker wanted to, with enthusiasm, imprison and torture (probably for information but perhaps just for fun) the old lady that was the subject of the conversation.