[4] Gemlik was the site of the ancient Greek city of Cius[5] (/ˈsaɪəs/; Greek: Kίος or Κῖος Kios), later renamed Prusias on the Sea (/ˈpruːʒəs/; Latin: Prusias ad Mare) after king Prusias I of Bithynia, was an ancient Greek city bordering the Propontis (now known as the Sea of Marmara), in Bithynia and formerly in Mysia (in modern northwestern Turkey), and had a long history, being mentioned by Herodotus, Xenophon, Aristotle, Strabo and Apollonius Rhodius.

It was at the foot of Mount Arganthonius, and there was a myth that Hylas, one of the companions of Heracles on the voyage to Colchis, was carried off by the nymphs when he went to get water here; and also that Cius, another companion of Heracles, on his return from Colchis, stayed here and founded the city, to which he gave his name.

The Cius may be the channel by which the lake Ascania discharges its waters into the gulf of Cius; though Pliny speaks of the Ascanium flumen as flowing into the gulf, and we must assume that he gives this name to the channel which connects the lake and the sea.

As Ptolemy speaks of the outlets of the Ascanius, it has been conjectured that there may have been two, and that they may be the Hylas and Cius of Pliny; but the plural ἐκβολαί does not necessarily mean more than a single mouth; and Pliny certainly says that the Ascanius flows into the gulf.

The position of Cius made it the port for the inland parts, and it became a place of much commercial importance.

Pomponius Mela calls it the most convenient emporium of Phrygia, which was at no great distance from it.

It joined the Aetolian League, and was destroyed by Philip V of Macedon in the Second Macedonian War (200–197 BCE), and given by him to Prusias I of Bithynia.

It was sometimes called Prusias ἐπιθαλασσίη, or "on the sea," to distinguish it from other towns of the same name, or πρὸς θάλασσαν.

At first a suffragan of Nicomedia, it soon became an autocephalous archdiocese in the Byzantine Empire, being listed as such in Notitiae Episcopatuum from the 7th century onward.

No longer a residential bishopric, Cius is today listed by the Catholic Church as a titular see.

In 1891, 5147 people lived in the town center including; 4620 Greeks, 242 Turks, 178 foreigner and 107 Armenian.

Karsak Stream, which originates at Lake Iznik, divides this plain and town center into two parts.

The Gulf of Gemlik, often called fig port of Kilyos (in reference to the ancient name of the town), is 35 km from east to west, 10–15 km from north to south, and 100–150 meters deep.

The largest river in the area is Karsak, gurgles from İznik Lake and spilles to Gulf of Gemlik.

In rural areas of Gemlik District mostly covered by olive trees and pines on the Samanlı Mountains.

There are 35 neighbourhoods in Gemlik District:[9] Gemlik has a hot-summer Mediterranean climate (Köppen climate classification: Csa) with cool and rainy winters, and hot and dry summers.

There are also smaller resorts like Büyükkumla, Karacaali and Narlı along the north coast of the gulf.

There is also a moto-cross tournament organised at Şahintepe point on the Samanlı Mountains and Olive Festival at city in August–September.

Industrial establishments that produce plastic, textile, marble, chemistry, machinery, automotive, steel and fertilizers continue their activities in the district.

Large-scale industrial facilities in Gemlik are concentrated on the western coastline of the district, especially in and around the free zone.

Bean, artichoke, cucumber, tomato, pea, eggplant, pepper are the leading vegetables grown.

When Free Zone Trade data is analyzed, in the same period, from abroad to the region in 2017, 430.902.457,11 TL.

In the district, besides large industrial facilities such as the under construction TOGG Turkish national car factory, Borçelik, Borusan, MKS Marmara, Çimtaş, Gemlik Gübre, there are many international organizations such as Ficosa, Yazaki, Aunde Vibracoistic, Techno Stell, Summit Stell, AB Rothec.

A total of 25,000 people work in the free zone, of which 15,000 are employed, and other industrial facilities are 10,000.

Cigarette inspections are carried out periodically by the related commission and in line with the incoming notifications.

Administrative fines related to smoking are given to business owners who act against the legislation.