What is a gm81 file and how do I open a gm81 file?
GM81 is a project file created by Game Maker, a video game development application that enables games to be created using drag-and-drop actions. It may include source code as well as references to game resources, such as graphics, sound effects, and music. Once development is complete, Game Maker projects can be built into a standard Windows executable EXE file.
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How to open gm81 file on Wikipedia
How to open gm81 file on Microsoft.com