At this point, Google has nearly 30% of the personal email market. That's a lot! Several years ago, in an attempt to reduce complaints about all the marketing email, Gmail introduced tabs for Social, Promotions, and Primary (the inbox). And of course, they still have the spam folder, buried somewhere no one every looks. Then, they use their ever-changing algorithms to route email to their appropriate category.
Now to say that email that ends of in the Promotions tab will not be seen is not true. There are three main reasons.
Number one is that Gmail does an excellent job of categorization and it's increasingly infrequent that spam email that users really just don't want even makes it to the Promotions tab. Those emails are routed to spam, never to be seen. That makes the Promotions tab not quite the black hole that it was a few years ago.
Number two is that Gmail does a very good job of making email a positive experience. And that includes marketing email. So more and more people are checking the Promotions tab for emails they signed up.
Finally, Gmail mobile doesn't have the tabs (at least not by default). For most, everything just gets routed to the inbox.
Still, email marked as a promotional email is 20-25% less likely to be acted upon than one that just makes it's way to the inbox. Reasons for this is that some people do turn tabs on the mobile client. Also, 40-50% of all email is still read or acted upon on the desktop.
So how do you avoid the Promotions tab?
Keep your emails simple. Avoid an excessive amount of images, links, or html markup.
Avoid clearly promotional terms like 'give now', 'donate today', 'urgent' in your subject line.
Be sure your email is sent from a domain that has been authenticated
This list isn't comprehensive and it's not trying to be since it will be outdated soon as Google is always changing their algorithms to be better at routing email.