A interesting topic!
While it is generally legal to decompile software for personal research or modding, most GameMaker End User License Agreements (EULAs) prohibit unauthorized reverse engineering. These tools are often used legitimately to recover lost source code for personal projects when original files are corrupted. GM8Decompiler/README.org at master - GitHub gamemaker 8 decompiler link
Legacy GameMaker executables consist of two primary components: The Runner: A interesting topic
Fixed conversion for better performance on new GPUs. gamemaker 8 decompiler link
(the hierarchy you see in the GameMaker IDE) is typically lost and must be manually reorganised. Version Compatibility