No, GameMaker Studio 2, the latest version, is not open source. It is a proprietary piece of software developed by YoYo Games. Users can purchase licenses to use GameMaker Studio 2 for game development, which comes in different tiers depending on the user's needs, such as the Creator, Developer, and Console licenses.
The source code of GameMaker itself is not available for users to view, modify, or distribute. Those interested in developing games with GameMaker must use the tools and scripting language provided (GML - GameMaker Language) within the constraints of the platform.
For developers seeking an open-source alternative, there are other engines like Godot, which offer similar functionality and are fully open-source, allowing developers to contribute to the engine's development or customize it for their own needs.