Can Gregorian Chant be sung in harmony?
Sure. We even have a name for the earliest harmonization of chant: organum. This seems to date back to the eighth century, right to the beginnings of the era when our Mass propers were written. Most of the liturgy was sung in unison right up until the end of the Middle Ages, but harmony was added for feast days and for high points of the liturgy. Today one can experiment with singing some chant in the style of early organum by adding a second voice at the interval of a fourth or a fifth above or below the melody. Another possibility is having low voices sing a sustained pedal point (the final note of the piece gives you the pitch), or this note along with the note a fifth higher. It is also possible to combine a pedal point (or two) with a harmonized melody-experiment and see what you like!
