1. Reach customers with specific audience profiles: You can target your customers in the following five ways
1) Age/sex
2) Geographical area/language
3) Device
4) Topics/interests (affinity audiences)
5) Keywords/Domains:

The method of using domain targeting can be used to target users who subscribe/receive emails from a particular company. Advertisers can add the targeted domain name to their ad keywords, for example, adding example.com to the keywords.

Note that the targeting methods related to audience characteristics are based on data that is publicly available to Gmail users (using Gmail to register for a site) or inferred by the system. Other targeting methods are based on the most recent non-spam content received by the user.

2. You can add Gmail ads to newly created or existing display campaigns. (We recommend you to create a new campaign for Gmail ads to manage your ads more easily)
3. In the Ads group, add mail.google.com targeting display location for your Gmail ads so that your ads will appear in the mailboxes of Gmail users.
For Gmail please select the "Target and bid" option in the targeting criteria options.
4. Gmail ads have an optimal display frequency. The frequency cap sets the maximum number of times an ad can be shown to a user (up to 4 times per week per user).