Hi, I'm using GMS2 (Game Maker Studio 2) and I'm wondering how I'm able to make it open source. I'm aware that GMS2 allows you to save the project as a Game Maker Project File where others can view the source code if they have the engine already installed. However, I'm assuming that most of the people here don't have GMS2 on their systems so, how do I make my project open source? 

GM Project Files are plain text. Others don't need to have GMS2 installed, so just push your code to your repository on GitHub and you'll have it open source. Others can easily view your code from there.