Why wouldn't I know that?
The agnosticism isn't total. There's no "nothing can be known for certain" to it, there's no "no conceptual abstractions are workable" to it. I'm agnostic about metaphysical declarations like "no sorts of gods are possible".
Like I said before, the point is "avoidance of totalizing claims".
If your brain imagines there's an implicit "all" (like "[total] avoidance of [all] totalizing claims") in things I say, you will have twisted them into something I haven't said and caused your own confusion.
How about "The only sorts of gods that are possible are those that are so poorly defined as to be meaningless, or so ineffectual to human affairs as to be irrelevant"?
Because I am certain that that's a true statement.
As a gnostic atheist, I do not consider wildly speculative 'god concepts' (that no person actually worships, and which exist only as thought experiments by wannabe philosophers desperate to insert doubt) to be gods at all, and nor do I credit entirely natural phenomena with the title 'god'.
Of course gods exist, but only in the exact same way that Superman and Spiderman exist - as fictional characters, usually with physically impossible attributes.
Superman is more real than Allah (for example), because everyone knows what Superman looks like.
Superman is even one up from Jesus, whose appearance (and, for that matter, whose godhood) are the subject of heated debate.
There are no gods. I know that there are no gods. I know this with the same degree of certainty that I know Superman isn't a real individual, and that the Moon isn't made of Stilton.
But if I say "The Moon is not made of cheese", I don't get dozens of keyboard warriors trying to bring ultra pedantic philosophy to bear and to persuade me that I can't really "know" these things for certain.
I can; And I do. The evidence includes (but is not limited to) the Standard Model of Particle Physics, which is the best tested and most accurate theory in the history of science to date.
I am, with good reason and total justification, as certain that no gods exist, as I am that this forum exists and that I am posting on it.