GDevelop is an open-source, cross-platform game creator designed to be used by everyone. Download it on gdevelop-app.com if you don't have it yet!
Using the game engine in your game
When you're developing a game with GDevelop, your game is exported with the game engine and events of the game are using it.
Development of the game engine (advanced)
If you want to work on the engine directly, follow the GDevelop 5 README about development of the game engine to get started. Refer to this documentation to know more about the engine.