This is an issue for all email services as Gmail uses very advanced and complex algorithms to review the mailing and determine if it considers it "Conversational" or "Promotional" content before sending it to what it considers the appropriate inbox tab.

If recipients decide to use the tabs feature in Gmail, there is no foolproof way to stop this from occurring but these articles give some insight into how you may be able to avoid it.

Both MailChimp and Constant Contact have articles addressing the issue:
MailChimp Article: About Gmail Tabs
Constant Contact Article: Email Delivery into Gmail's Primary Tab or Promotion Tab

The following Article on Quora gives some actionable tips that can be taken to make your mailings look less like a promotion to Gmail:
How can I get Gmail to stop filtering my emails as promotions? I'd appreciate any information that anyone has, including information about how I can contact Gmail about this issue.

In addition, you can try creating an SPF record by following our KB article: What is an SPF Record? and/or request a DKIM Key that is used to digitally sign emails sent from NetCommunity by installing it on your DNS using a TXT record.

Even after following all of the recommendation in this article, you may still find your emails being sent to the Promotions tab in Gmail.

There is no sure fire way for you to ensure your emails are not sent to the Promotions tab but these tips will give you some guidance.