A gameplay design developer is responsible for creating the interactive aspects of a video game, ensuring that the game is engaging and enjoyable for players. They devise the rules of the game, design game characters, and develop game scenarios and storylines. They also determine difficulty levels, create game interfaces, and define player progression. They work closely with other game developers like programmers and artists to implement their designs into a playable game. Additionally, they test and refine the game based on feedback. Their ultimate goal is to create a compelling game experience that meets player expectations and commercial objectives.