Derailleur hanger marked as GW-9L is a bike component specifically designed for mounting on frames made of aluminum. It's a crucial part that facilitates the proper functioning of the bike's derailleur. Responsible for attaching the derailleur to the bike frame, it ensures stability and precise operation of the derailleur mechanism. The GW-9L number indicates its specific compatibility with certain models of aluminum bike frames. It's important to select the appropriate derailleur hanger for the specific frame model to ensure proper derailleur performance and compatibility with the bike frame.