GMail disagrees with you, because GMail users disagree with you. They are clicking "report spam" on your emails. Whether or not you think what you're sending is spam, the recipients think it is, and that's what matters. (Based on the other comments in this thread it's not hard to see why they might think so.)
This is not always the case. Last week, my monitoring system sent me 20k emails in a few hours in response to a server attack. When those hit my gmail inbox, gmail marked them all as spam. Myself, the user, did not mark them as spam. Gmail did that for me. But their reputation system is behaving as if 20k people marked 20k emails from us as spam. In response to those 20k emails marked as spam, now our domain sender r…
I check my spam folder regularly and it has been this way for as far as I can remember.