To Send an email using the gmail integration, you firstly need to make sure that you've linked your account with Google via the integration (for information on how to integrate with google, click here)
Then, you can go to the main menu on the left hand side and click "Messages" tab (if this tab is missing, please contact support to discuss your account)
Once clicked, you will then be taken to the Google integration for the inbox. Here you can access the different mailboxes, etc as well as the option to click on "New EMail"
Clicking on the New Email tab will bring up the normal blank email that you would normally get with Gmail. Here you can select who you want to send the email to - typing in a partial search of the contact will look in your system and find the contact you wish to send the email to. Also clicking on the little downward arrow will open up the option to add a CC or BCC to your email before completing the rest of your email and sending
By sending an email this way, the system will automatically log it against the person you are sending the email to