"I've been getting lot of promotional emails recently which fall into my Gmail inbox (rather than spam folder) hence triggering a notification. I've never subscribed to these emails." - how do you think, where do they get your info from? why are they spamming you (in your opinion)? I would suggest you to hide your email somewhere on your site/blog. And make it look it like "name [at] gmail [dot] com". They would need to find your email before spamming you

Been using the same address for 8 years so I think maybe it's been in many email-dumps which are usually sold for email marketing campaigns.

Replying to the email can itself trigger further spam because it just proves there is someone home at your address. I know you don't want to change providers, but essentially you're asking for a feature (spam reporting, phishing reporting, hands-off unsubscribe) that your provider doesn't have. Moving from Gmail is awful but it can be worth it for features like that and others related to security (SPF, DMARC) and privacy (suppressing inline images, showing full link URls), etc.

Using the gmail web app, click on the spam email and then click on the vertical elipse ... in the upper right-hand corner of the email. Select "Report spam" to make it and that sender disappear forever. You may have to do this a few times, but persevere... you are doing the world a favor by training the spam filters.

There are well-known reputable organizations in the U.S. that spam me because someone else entered in my email address and those organizations never validated the email address.

I mark them as spam, but it takes a long time for them to start going to the spam folder instead of the inbox.

I'm sure there are users here that are employees of these organizations/companies/etc. Please make sure your organization is validating emails! 'Never trust the user' should extend to email addresses too.

this has been my approach so far.

Look in the e-mail headers to see if it comes from a reputable e-mail service or marketing provider. If so, notifying them should make them suspend the customer's account as they wouldn't want to be flagged as spam (as that would ding the reputation of their e-mail servers and affect their other customers too).

That what I was hoping to do but I believe it's a manual process and would take some time. I Wish it was that simple using some extension or some other service.

I would assume you could just block the email addresses and/or the domains?

Unless they are genuine spam from those random spammy mail addresses, then I’m surprised they are passing through the spam filters