GMK Assembler is used to decompile Game Maker game projects into separate data files and allows you to rebuild your project from them. The current version supports GM file versions 8.0 to 8.1.71. So far it only allows exporting/importing XML files in "Accurate" mode. Project is open source, released under GPL license.
requires .NET Framework 4.0; the installer should detect if you have it and allow you to download if you don't, but you can also download it here: https://www.microsoft.com/en-us/download/details.aspx?id=17113