GameMaker Studio lets you start creating your game from scratch. It has two options: Drag-and-Drop, and Game Maker Language. The first one is by far the easiest, since you only need to select the elements you want to include in your game, and drop them into the workspace. The second one needs you to have a solid programming knowledge... (
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The program works with the following extensions:
gm6,
gmk and
gmd