G.K.'s comment is more of an “off the cuff” comment than anything else, so I’m not sure all of our conclusions can really be extrapolated logically.
However, let’s get a little deeper here because there are some implicit results that can be drawn from what he says. Does “If there were no God, there would be no atheists” imply that atheists would not exist if God didn’t? I’m not so sure. Not that it would be a good world obviously, but imagine a world in which the resounding theory was of an anti-God — for example, the majority of people worshipped the lack of a God. I know it might sound like a nightmare, but in this world the minority might believe in God. In which case, the comment would be something like this:
“If there was a God, there would be no believers.”