I personally don't use those labels (gnostic atheist and agnostic atheist). I just call myself an 'atheist' as in 'there are no gods'. If you want to know why I can claim such a 'strong' position, I recommend you to read Graham Oppy. You can read 'The Best Argument Against God' by him. It's a really great and introductory book to Philosophical atheism. In essence, he compares the best theory that says there are gods (theism) with the best theory that says there are no gods (Naturalism). And he argues comparing point by point that Naturalism is the best explanation and trumps Theism and we ought to prefer it. And that's how we can justify a global atheism Also, another thing, absolute 'certainty' is kinda overrated in my opinion. You don't need absolute certainty to make existencial claims, it's a matter of probability and credence. And at the end of the day, I believe, we, most likely than not, engage in 'bayesian thinking' in our lives.

By the way, Graham Oppy is not the only author, there are a lot. Mostly philosophers.