By default, outbound email notifications are sent from Zendesk's servers. However, if the Send email via Gmail setting is enabled (Admin > Channels > Email), outbound notifications are sent directly from the Gmail account that you've configured as a support address.
This applies only to tickets that were received at an external support address connected via the Gmail Connector.
Because the email was sent from your Gmail account, you can check the Sent folder in that account for the email in question. If your Gmail account indicates the message was successfully sent, but the recipient did not receive it, share the message ID in the email header with the recipient's email admins for investigation.