A Google Tag Manager developer manages and deploys marketing and analytics tags on a website or mobile app. They work on a tool called Google Tag Manager, which allows for quick updates and streamlined tagging process. Their tasks include creating, testing, and implementing tags, triggers, and variables. They also configure third-party tracking tools, set up event tracking for user interactions, and ensure data accuracy by debugging and fixing any issues. Furthermore, they collaborate with marketing and development teams to understand tracking requirements, facilitate data tracking and analysis, and improve overall digital marketing performance.