As an AI language model, I can help guide you on how to create a simple HTML game based on the Avatar movie in a GTA-style galactic adventure. However, creating a full-fledged 3D game requires advanced programming skills and is beyond the scope of a simple text-based conversation. Nonetheless, I can provide you with a basic structure and implementation to get you started. Here's a simplified example:
In this basic template, the script tag encloses the JavaScript code responsible for initializing the game, setting up the canvas, and starting the game loop. You can build upon this skeleton by adding game logic, assets, characters, and implementing game controls using JavaScript and additional libraries such as three.js for 3D rendering.
Keep in mind that creating a realistic 3D game like GTA requires extensive knowledge of various programming languages, frameworks, and tools. It would be helpful to explore specific game development resources and tutorials to delve deeper into the topic.