Using Gmail integration options 

Calendar integration helps you sync specific events and emails to Salesforce. This is useful so that you don't create double entries in your email/calendar client and Salesforce. The following options are available for Gmail:

• Send through Gmail: This is for representatives who spend most of their time in Salesforce but want to use a Gmail account to send emails. Emails are composed in Salesforce but are sent through a connected Gmail account and appear in the Gmail Sent folder. When Send through Gmail is enabled, the activity history for leads and contacts includes a Compose Gmail button. Emails are logged to the records the email was sent from.
• Gmail Integration: This is for representatives who spend most of their time on their Gmail account. When emails are sent, they can select which Salesforce records to log the emails to.
• Einstein Activity Capture: This option is for users who prefer to have emails logged automatically. Einstein logs email activity from a connected client or device, including Gmail.