Thanks Cpl.Bator, I've added GD to the list
Nexus, there is (a quite old and) short video here:
or the beginner tutorial:
I should create a new video in the next few weeks to promote GD
MorleyDev, part of GD are already open source, you can browse the repositories here :
https://github.com/4ian/GD-Extensions and
https://github.com/4ian/GDJSCompared to C2, GD is available on Ubuntu (GD is made using cross platform technologies), support native games development, events are translated to C++ or JS (should be more efficient and games can't be reversed engineered). Some features are also designed to be more powerful (variables support the use of arbitrary structure, for example any variable can be parsed to/from JSON).
Concerning Game Maker, both C2 and GD are based on events which I think is a better way of creating games without asking the user to learn a new (and proprietary) language like the GML. Game Maker drag'n'drop approach is more limited and not as powerful.