Why I am a gnostic atheist (gnostic nullifidian)

Here's the way I like to put it:

Yes, I understand that absolute truth is impossible to possess, and that anyone who claims to have absolute knowledge is a fool. However, I do claim to know there is no god, and by the same merit, nothing else metaphysical going on.

Living in reality, we are never capable of declaring anything absolute fact. However, we often do declare things facts. From a scientific standpoint (and a practical one as well), a fact can best be defined as a hypotheis that is accepted as true without exception because the probability of that hypothesis being correct is so high that we can't be bothered to worry about the negligible chance that it's not. For all practical purposes it is "true".

Now let's apply this to God. True, we don't know the odds of God being real, but we do know of things we are certain are not real, like unicorns, elves, leprechauns, and (in the case of theists) the gods of other faiths. For our practical purposes, we assign the hypothesis, "Unicorns do not exist," 100% probability. Now, I present a question: What is there that would suggest in any way that the odds of God being real are significantly different from the odds of any of these being real?

I don't think that there are any valid arguments for God being fundamentally different from any of these other assertions. Therefore, I am led to assign the assertion that God is not real a probability of 100% as well. By this logic, I can, in fact, declare God's nonexistence to be a fact, and thus claim to know it, without having perfect knowledge.

This is gnosis, and at the same time acknowledges the imperfection of human knowledge.

The only possible counterargument to this logic, I believe, would be to somehow prove that I am underestimating the possibility of unicorns, leprechauns, etc. being real, claiming that this too is an unknown probability (despite having an accepted value of 0%).

Thoughts, r/atheism?