Commentaries and other reference materials are displayed alongside the sections of Scripture that they discuss. To access commentaries for a particular Bible passage, look up the Bible passage you have in mind (for example, John 3:16). Then look for the the blue box labelled Study This. Click anywhere in the blue box to open related commentaries and other resources:
Clicking on the blue Study This button opens a window that shows all commentary related to the passage you're looking at. Click on a type of reference (e.g. "Study Bibles") to see a list of available reference works of that type. References marked free are freely accessible to anyone; other reference works require a Bible Gateway Plus subscription to access:
To read a piece of commentary, simply click on its title and it will be displayed without disrupting the Bible passage you're reading.
To hide the reference window, simply click on the blue box again, and the Study This panel will close. You can access it at any time by clicking Study This again.