In Salesforce Classic, you can integrate Gmail to Salesforce in the following ways:
Gmail to Salesforce
Gmail to Salesforce lets you log emails automatically. It also lets you log chats you send from your Mails account as activities on leads, contacts, opportunities, and other records that support activity history.
It’s important to know that after you activate Gmail to Salesforce, you can’t deactivate it.
To activate Gmail to Salesforce
Gmail Buttons and Links
Gmail Buttons and Links adds Gmail links next to email fields on all records and adds Compose Gmail buttons in Activity History on leads and contacts.
When you select a Gmail link or Compose Gmail, Salesforce automatically logs you in to your Gmail account and automatically populates the To field. If Gmail to Salesforce is activated, Salesforce also populates the BCC field with your Email to Salesforce address.
To activate Gmail Buttons and Links:
This blog information is very useful to learn about Gmail to Salesforce Integration.