gl_FragCoord
is in window space. And window space is in pixel coordinates of the window. So virtually all of your fragments are going to have values > 1.0. And since you're almost certainly not rendering to a floating-point framebuffer, your colors will be clamped to the [0, 1] range.
gl_FragCoord.z
is probably not 1.0, but it may be close enough to it, depending on your depth range and how far the objects are from the camera. If you want to get a real idea of the depth, you need to linearize it.