He was one of a small group of scholars who invigorated medieval Europe in the twelfth century by transmitting Greek and Arab traditions in astronomy, medicine and other sciences, in the form of translations into Latin, which made them available to every literate person in the West. One of his most famous translations is of Ptolemy's Astronomy from Arabic texts found in Toledo. Gerard has been mistakenly credited as the translator of Avicenna's Canon of Medicine (see below).
Toledo, which had been a provincial capital in the Caliphate of Cordoba and remained a seat of learning, was safely available to a Catholic like Gerard, since it had been conquered from the Moors by Alfonso VI of Castile. Toledo remained a multicultural capital. Its rulers protected the large Jewish colony, and kept their trophy city an important centre of Arab and Hebrew culture, one of the great scholars associated with Toledo being Rabbi Abraham ibn Ezra, Gerard's contemporary. The Moorish and Jewish inhabitants of Toledo adopted the language and many customs of their conquerors, embodying Mozarabic culture. The city was full of libraries and manuscripts, the one place in Europe where a Christian could fully immerse himself in Arabic language and culture.
In Toledo Gerard devoted the remainder of his life to making Latin translations from the Arabic scientific literature.
Gerard edited for Latin readers the Tables of Toledo, the most accurate compilation of astronomical data ever seen in Europe at the time. The Tables were partly the work of Al-Zarqali, known to the West as Arzachel, a mathematician and astronomer who flourished in Cordoba in the eleventh century.
Al-Farabi, the Islamic "second teacher" after Aristotle, wrote hundreds of treatises. His book on the sciences, Kitab al-lhsa al Ulum, discussed classification and fundamental principles of science in a unique and useful manner. Gerard rendered it as De scientiis (On the Sciences).
In total, Gerard of Cremona translated 87 books from Arabic, including Ptolemy's Almagest, al-Khwarizmi's On Algebra and Almucabala, Archimedes' On the Measurement of the Circle, Aristotle's On the Heavens, Euclid's Elements of Geometry, Jabir ibn Aflah's Elementa astronomica, the chemical and medical works of al-Razi (Rhazes), the works of Thabit ibn Qurra and Hunayn ibn Ishaq, and the works of al-Zarkali, Jabir ibn Aflah, the Banu Musa, Abu Kamil, Abu al-Qasim, al-Farabi, al-Kindi, and Ibn al-Haytham.
Other treatises attributed to the "Second Gerard" include the Theoria or Theorica planetarum, and versions of Avicenna’s Canon of Medicine— the basis of the numerous subsequent Latin editions of that well-known work — and of the Almansor of al-Razi ("Rhazes" in Latin-speaking Europe). The attribution of the Theorica to Gerard of Sabbionetta is not well supported by manuscript evidence and should not be regarded as certain.