In the Gmail app, you have two options:

1. Report an email as “not spam” (if the email is in spam).

If an email is ending up in your spam folder, open the email and click Report not spam.

You can also tap the three dots in the upper right corner and click Mark important to give it priority in your inbox (see the video to the left).

2. Move an email from the promotional inbox to the primary inbox.

To move an email from the promotional folder to your primary inbox, open the email and tap the three dots in the upper right corner, then tap Move to > Primary. (see the video to the right).

Adapted from: https://sumo.com/stories/whitelist-email?ref=em-se-071919