Here is a list of all files with brief descriptions:

renderer_GL_common.inl
renderer_GLES_1.c
renderer_GLES_2.c
renderer_GLES_3.c
renderer_OpenGL_1.c
renderer_OpenGL_1_BASE.c
renderer_OpenGL_2.c
renderer_OpenGL_3.c
renderer_OpenGL_4.c
renderer_shapes_GL_common.inl
SDL_gpu.c
SDL_gpu.h
SDL_gpu_GLES_1.h
SDL_gpu_GLES_2.h
SDL_gpu_GLES_3.h
SDL_gpu_matrix.c
SDL_gpu_OpenGL_1.h
SDL_gpu_OpenGL_1_BASE.h
SDL_gpu_OpenGL_2.h
SDL_gpu_OpenGL_3.h
SDL_gpu_OpenGL_4.h
SDL_gpu_renderer.c
SDL_gpu_RendererImpl.h
SDL_gpu_shapes.c