Yeah, the Scooby movie was on Cartoon Network last weekend, and I did a double take when I saw Gunn's credit. Turns out he did stuff for Troma even.

You can't always predict what someone will achieve based on their past work. If somebody had told me back in the early 90's the director of Bad Taste and Meet The Feebles would go on make the LOTR trilogy, I would have had my doubts. ;)

Movie scripts are frequently rewritten by other people though. Unions and studio politics often dictate who gets final credit, or blame.