I literally missed an important webinar due to this shitty Gmail
If anyone have an idea why Gmail behaves like this... I really want to know

All sorts of factors go into this, but Google keeps their algorithm close to their chests.

Configuring DNS correctly, using correct headers, avoiding keywords, not sending email too often, not sending email not often enough.

That all matters.

Probably something as simple as enough other people marking mails from the same source as spam...

I've used a lot of mail services but can confirm that Google's spam detection, while not perfect is the best I've encountered so far.
Things that matter, SPF, DKIM and DMARC setup properly. Of course spammers that own the domain can also do this so if the company sends aggressive marketing their recipients mark it as spam and are fucked.
If a service is used it can have some issues with the the IP being abused by other users of that service and your mail is collateral damage. Though this I've mostly experienced with MS mail recipients (live, outlook, hotmail accounts).

@hjk101 a mail from qwiklabs (I think it has some partnership with Google) was hiding in spam