Gerace was founded by refugees. The people of Locri Epizefiri withdrew from the coast between the end of the seventh century and the beginning of the eighth, driven off by Saracen raiding, and established a new settlement on this defensible height. They called it Santa Ciriaca , and the name appears in a document of 787 . The upper city keeps the Byzantine plan: a fortified castron with concentric walls and twelve gates.

The diocese moved with them, and its early history is unusually well attested for southern Calabria. The first bishop of Locri whose name is known is Dulcino , mentioned in a letter of Gregory the Great, who died in 594 ; Marciano followed from 598 , then Crescenzo (649), Stefano (680), Gregorio (died 709), Cristoforo (787) and Giorgio (870). In the eighth century the see was subjected to Constantinople and made suffragan of Reggio, adopting the Greek rite and becoming a home for Greek monks — among them, in the tenth century, Nicodemo , canonised by popular acclaim and still venerated locally.

The tenth century was hard. The diocese suffered continual Arab incursions, Gerace itself was sacked in 986 , and toward the end of the century its territory was reduced to make room for the new diocese of Bova .

In 1059 the Normans took Gerace under Robert Guiscard . They did not disrupt the diocese: the Byzantine rite was maintained and the Greek monasteries prospered more than before. What they did was build. The castle was fortified and the cathedral raised, and Gerace became a great building site. The first known bishop of the Norman period is Leonzio I , who in 1100 presided over the rebuilding of the monastery of San Filippo d'Argirò, and who may be the bishop under whom the cathedral works began in 1080 .

The Cattedrale di Santa Maria Assunta is the town's monument and one of the most important Norman buildings in Calabria — the largest Romanesque church in southern Italy, declared of national interest, and elevated to basilica minore on 8 September 2018 . Construction began in the late Byzantine period and finished under the Normans, which is visible in the projecting transept and the arrangement of the apses. Tradition, recorded on two plaques inside and in the Bollario of Bishop Ottaviano Pasqua at the end of the sixteenth century, gives its consecration as 1045 ; it was consecrated a second time in 1222 , most probably in the presence of Frederick II , then passing through Gerace.

It was built out of the city the founders had left. The twenty classical columns of the nave are reused from ancient Locri — from its temples and from the villas of the marina — with capitals partly ancient and partly remade. Beneath the transept lies a crypt on a Greek-cross plan with Roman columns, whose origins appear to go back to the primitive eighth-century place of worship, built directly on bare rock. Of the three apses only the northern one is original; the other two are fifteenth-century reconstructions by the Caracciolo counts after a ruinous earthquake. The Baroque high altar in polychrome marbles is the work of the Palazzotto brothers of Catania and the Messinese artist Amato.

The cathedral's later history is a sequence of damage, collapse and repair, culminating in the earthquake of 1783 and the abandonment that followed, then careful restoration. The Norman castle at the top of the town is now a ruin, destroyed by the successive earthquakes. The main street through the borgo is via Zaleuco , named for the lawgiver of the Greek city these people came from.