Gustav is a Red Iron mercenary tasked to kill Lord Harimann.
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. Hawkethen finds Gustav surrounded by Harimann's guards who interrogates Gustav. After 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.
Whatever Hawke chooses to do with Lord Harimann, Gustav returns to Meeran.