I am not familiar with geometry shaders but here are few thoughts:
- you are using two triangles with unappropriated culling, switch culling off if there is a second triangle shown (easy to debug)
- you are try to render triangles with triangle fan method, and output is expected to be two triangles. Add two more vertices, or render with triangle fan
Anyway 3d APIs usually work with triangles not quads. However, OpenGL have GL_QUADS rendering method, but that is not geometry shader.