"Okay, atheists say that God doesn't exist because he can't be seen."

No I say god probably doesn't exist because there's no evidence suggesting such.

"Well, gravity can't be seen, which is something atheists believe in"

There's plenty of evidence for gravity, such as letting something go any distance from the ground.

"Is it therefore illogical for atheists to not believe in God? Don't you all see the contradiction in atheists' thinking?"

Again, we have evidence for gravity, give me evidence for god.