What are some alternatives?
When comparing Unity and GameMaker, you can also consider the following products
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
CryENGINE - The most powerful game development platform is now available to everyone. Full engine source code.
Arkit - Discover a smarter, map-based project portfolio. Easier planning, better data management, great collaboration - Arkit gives you one intuitive system to make every day more productive.
Stencyl - Make iOS (iPhone/iPad), Android, Flash, Windows & Mac games without code using Stencyl.
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.