The project description.

Gray Game Development Kit is an ultra-fast 2D game engine for Windows 2000 or higher. It uses OpenGL for rendering.

The license.

The Gray Game Development Kit is distributed under a zlib license.

Features.

A wide range of the supported compilers.
The minimum number of external dependencies: DirectX, OpenGL, and the standard Windows API libraries.
You don't need to link a main program with the engine binary file. The engine is just the source code files.
The easy-to-use object-oriented API.
The sprites.
The backgrounds.
The tilemaps.
The image loader with TGA support.
The text drawing subsystem with custom font support.
The coordinate transformation.
The timers.
The collision detector.
The 2D camera.
The integrated FPS counter.
The powerful input subsystem.
The simple audio subsystem.