Which Italian opera librettist wrote the texts for many of Christoph Willibald Gluck’s early operas?

The Italian librettist who wrote the texts for many of Christoph Willibald Gluck’s early operas was Pietro Metastasio.1

While Gluck initially set many of Metastasio’s conventional opera seria librettos, he later became a central figure in an operatic reform movement that sought to move away from the elaborate conventions associated with Metastasian works. This reform was driven by a collaboration with the Italian poet and librettist Ranieri Calzabigi. Together, they created three significant "reform operas"—Orfeo ed Euridice (1762), Alceste (1767), and Paride ed Elena (1770)—which emphasized simplicity, naturalness, and emotional truth over the ornamental excesses of the era.1234