Gustav is Red Iron mercenary tasked to kill Lord Harimann.
Involvement
Gustav is involved in Loose Ends. He was sent by Meeran to kill Lord Harimann while he was preparing his cargo for shipping. After a while, Gustav and his men never came back to report to Meeran. This caused Meeran to hire Hawke to find Gustav. Hawke then finds Gustav surrounded by Harimann's guards who interrogates Gustav. After the Harimann's men were disposed of, Gustav tells Hawke that he was caught and his men killed. Lord Harimann then arrives and converses with Hawke.
Result
Whatever Hawke chooses to do with Lord Harimann, Gustav returns to Meeran.