The regular places Gunnar can only show up (and have sex with him) after progressing with the Bandit Quest and Northcrest Quest is the Fair, the Outdoor Fun, and the River, I guess. At least that's the only ones in my current playthrough. He also shows up for one time during the construction of More Houses in Bareshade but for regular appearances, the Fair, Outdoor Fun, and Riverside are the places I could only think of.