Even atheists don't -know- whether there's a god or not. So the statement Gates made sounds like the statement from a politically correct atheist. Einstein also considered the idea of a personal god very implausible.
I think an agnostic is somebody who doesn't know whether god exists, but considers it a 50/50 situation. Both sides are equally plausible. A fence-sitter, if you will. An atheist thinks extraordinary claims require extraordinary evidence, and that the evidence for god doesn't stack up. Therefore atheists are forced to conclude there probably is no god. Or simply said: there is no god. Even though we all admit we may be wrong.
I disagree with you on this point. Atheists make the claim that there is definitively no god, just as theists make the claim that there is one definitively.
All people I know who label themself atheist accept they might be wrong. I don't think atheists exist that claim to know with absolute certainty that no god could possibly exist.
In fact, I'm pretty sure not a single person reading HN is an atheist by your definition. Even Richard Dawkins is an agnostic by your definition. I can only conclude that your definition does't make sense.
Challenge: find me an atheist who fits your definition.
I'm not saying that they reject the possibility that they might be wrong; they couldn't do that any more than they could reject the possibility that they're wrong about anything immeasurable (to themselves).
The very definition of Athiesm is to deny the existence of a god or a higher power.
As soon as you admit there is a real possibility that there may be a god or a higher power; bam, you're an agnostic.
Which is the crux of my point - the definition of Atheist is often misused, as in this article. There are far more Agnostics than Atheists, it's just that Atheism gets used more frequently.
This page discussing the differences far more eloquently than I ever could:
I think an agnostic is somebody who doesn't know whether god exists, but considers it a 50/50 situation. Both sides are equally plausible. A fence-sitter, if you will. An atheist thinks extraordinary claims require extraordinary evidence, and that the evidence for god doesn't stack up. Therefore atheists are forced to conclude there probably is no god. Or simply said: there is no god. Even though we all admit we may be wrong.