I have read that there is a light extension but how can I add extensions? And can I add it to a already-existing project?
In GDevelop 4 you can enable the extensions by double click on extension in the project browser on the left and you should be able to see a list of extensions available and enable the ones you need. Note that the Light extension available for the native platform only and not for HTML5.
In GDevelop 5 it is no longer required, you have access to all objects and events out of the box except Light. There is no light object or extension in GDevelop 5 to my knowledge. Someone from the dev team did mentioned may going to look in to adding lights, I did not follow the development recently in case he did then you better ask on GitHub how to add. It is not available in a public release yet as far as I can see.
Oh ok. I thought they already added lights to GDevelop 5. Thank you for your answer