Gunther Weller is co-owner of Weller's Arsenal with his (assumed) wife Marda Weller. Gunther Weller is a level 30 weapons vendor. He is located inside Weller's Arsenal along with Marda Weller and Woo Ping. Weller's Arsenal is about 20 feet south-east, of the Stormwind auction house in the trade district.
The name could be a reference to characters Gunther von Kleist and Conrad Weller, both expert swordsmen from the Japanese anime Kyou Kara Maou.
See List of Stormwind NPCs.