Gainsight completed testing compatibility of our application with the Lightning UI in the Fall of 2016. Users shouldn't need to toggle between the Classic and Lightning modes in SFDC, but for informational purposes, we're still sharing the following instructions.

If you enable Lightning Experience...

If your organization has enabled the Lightning Experience, users can switch back to the Classic mode by navigating to the Switcher. How do you get to the Switcher? In Salesforce Classic, click your name in the upper right corner, and select Switch to Lightning Experience from the menu.

Navigate to the Switcher from Salesforce Classic

Navigate to the Switcher from Salesforce Classic

In Salesforce Classic, click your name in the upper right corner, and select Switch to Lightning Experience from the menu.

Navigate to the Switcher from Lightning Experience

Navigate to the Switcher from Lightning Experience

In Lightning Experience, click your profile picture in the upper right corner, and select Switch to Salesforce Classic from the menu.