In some cases, subscribers may report your emails are going to their spam folder, even if they have moved your emails to the Primary inbox in Gmail (or another email client).
The next step is to ask your subscriber to safelist your sending domain.
If they are using a corporate email address, they may need to ask their IT departments for help.
Safelisting will help your users ensure that your emails do not go to spam and improve your overall deliverability. You may want to include this information in your welcome email as well.