My campaigns land in Gmail's Promotions tab
Your campaigns landing in the Promotions tab is actually good news. The Promotions tab is part of the main inbox, so it doesn't indicate any issues with deliverability. The messages land there based on their content. The Gmail algorithm checks if your message is an advertisement, and if it decides that's the case, it will place the email there.
Google doesn't share the logic behind that algorithm, so there aren't any specific instructions on what should be changed in the message to hit the main inbox folder. Instead, convincing Google that people expect your campaigns there is better.
Although you might experience a decrease in open rates when your emails land in the Promotions tab, keep in mind that opens occurring in the Promotions tab are much stronger. An open in the Promotions tab means that your subscriber picked your campaign out of other promotions and wanted to engage with it. Since they were exploring their promotional emails, they are there for the shopping intent, making them more likely to follow the call-to-action (or make a purchase).
If people are excited to read your content, they will read it no matter what folder your campaign lands in. They might even move your email messages from promotions to the main tab to not miss them in the future. You can also ask your subscribers to move your emails to the main inbox if they find your campaign in other folders.
Here are some instructions from Google that you can share with your engaged Gmail subscribers to help them move messages to the primary tab. The more people do that, the bigger chance for your emails to land there directly.
However, please remember that it's better to embrace landing in the Promotions tab rather than trying to avoid it, as it might have the opposite effect on your overall deliverability.
Instead, we recommend that you build your sender reputation because it will help you achieve better inbox placement and so might help you land in other folders than the Promotions tab.
You might also want to test different subject lines, see if there are not too many images in your message and avoid using words such as "sale", "buy", "offer", etc. within the content. We would suggest finding ways to make your interactions with your audience feel more personal as that will not only help you look less promotional to Gmail, but your subscribers will likely enjoy the relationship-building email as well.