Cultural foundations of Gaya had been laid out since the first century B.C., and its culture prospered in the third and fourth B.C. Gaya had been a confederacy of small polities in Nakdong River basin of southern Korea for 600 years before its demise in the year of 562. Gaya boasted a flourishing iron culture, earning itself the name of Kingdom of Iron. High quality iron of the times was produced in the present Gimhae area. And, Gaya built up national strength using its stellar mastery of iron.

Its influence could be felt in as far as Silla and Japan culture-wise. In those days, Japan had to depend on iron made by Gaya entirely since it didn't have any skill regarding iron making. As Gaya provided raw iron and advanced iron-making skill to small states of Japan, it held mighty influence over Japan in exchange for helping it develop its own iron culture. Iron manufactured by Gaya was imperative in supplying iron to East Asia.

Through a port blessed with natural advantages in then-Gimhae Bay, presently known as Gimhae Plain after the sea dried up, Gaya enjoyed its heyday with its expertise in intermediary trade by exporting iron to Silla, Japan and China and created a unique culture by embracing diverse cultures of others.

The first feature of Gaya culture. Delicate and elaborate craftsmanship Gaya culture was less flamboyant than that of Silla and was distinctive in that it made use of much glass and jade instead of gold and silver. Its cultural heritage, such as earthenware, helmet, armor, saddlery with which people of Gaya used to ride or manage a horse as well as gold crown, features sharper and more delicate craftsmanship compared to Silla. When an old tomb from the latter days of Gaya was unearthed, Gaya's advanced iron making and mastery became pronounced through other excavated goods such as iron arrowhead, iron helmet, sword and other weapons made of iron as well as helmet, armor and accessories.

The second feature of Gaya culture. Creative earthenware and iron culture Gaya's earthenware was made of sand-free mud and created employing a novel method of using a spinning wheel. Such method influenced the way Japan made its earthenware. Its representative piece of work is a vessel in the shape of a warrior on horseback, which was estimated to be made in around the 5th century. This invaluable vessel says much about how advanced Gaya was in terms of making earthenware.

Another artifact is a vessel in the shape of a shallow-draft boat with a flat bottom. Its color, feel of material and patterns embody characteristics of Gaya vessels well. Especially, Wooreuk created Gayageum or a twelve-stringed Korean harp and handed it down to Silla where his music became central to Silla's court music. This clearly shows how advanced Gaya culture had been. After the demise of Gaya, its people were admitted to Silla while members of the royal family Kim of Geumgwan Gaya became Jingol noble classes and Kim Yu-shin of Gaya descent contributed to the unification of the three kingdoms by Silla. After his death, the general received the posthumous appellation of King Heungmu the Great and was revered as one of Silla kings. Descendants of the royal family Kim of Gaya live by one of major family names in Korea, Kim, especially those Gimhae Kim clan.

Controversy over a Japanese outpost in Imna(Mimana)
A theory that Gaya was a tributary of Wae of northern Kyushu in Japan argues that Japan conquered the southern tip of the Korean peninsula, then Gaya, where it established a Japanese outpost. In particular, there are even records of King of Japan reigning over Silla, Baekje and Goguryeo through this outpost. This theory was fabricated colonial history with which Japan used to justify its invasion and colonization against and of Joseon. However, the theory is still deemed as the truth by Japanese of today and is used with educational purposes to justify Japan's colonization of Korea.

The first controversy – Did a Japanese outpost exist?
In conclusion, the outpost did not exist. In the chapter on Gangsu in Samguksagi, historic records of the Three Kingdoms, there is a part that mentions the area called 'Imna Garyang'. It is easily apparent that Imna is Geumgwan Gaya considering the monument erected at a tomb for the Great Buddist Monk Jingyeong that mentions of Kim Yu-shin, also posthumously known as King Heungmu the Great, of the Imna royal family descent. Regarding the theory of Japan establishing an outpost in Imna, Korea-Japan Joint History Research Committee agreed recently that the outpost did not exist and, in turn, the Japanese academic world has finally negated the theory completely.

The second controversy. Nihon shoki – The conquest of the three kingdoms by Empress Jingu In Nihon shoki, also known as the Chronicles of Japan, a fictitious character appears as Empress Jingu. It is written in the Chronicles that the Empress conquered Gaya in the year of 369, established a Japanese outpost in Imna, reigned over it as its tributary and fell to Silla in the year of 562. The Chronicles are now suspected of having made up the myth about Empress Jingu in order to strengthen royal powers of the King at that time. Details of the Chronicles seem to be a patchwork of historical records written by Baekje, which had been arbitrarily modified to serve intentions of Japan. Moreover, the reality is that not only Korea and China but also Japan does not agree to literal translation of the Chronicles.

Archeologically, a theory of Empress Jingu of Japan conquering Gaya in the 4th century is false. Back then, Gaya had advanced military culture under which it managed organized army equipped with diverse saddlery and plate armor that are produced using its iron making skills. By comparison, Japan lagged far behind Gaya since it could not even make a smelting furnace for iron making until the early 6th century. Also, there is no evidence of Japan managing organized army at this time since only rudimentary weapons, such as daggers or thin double-edged swords, were used. Thus, most of scholars discount the theory of Japan making Gaya as its tributary and of Empress Jingu conquering the Three Kingdoms as only a myth.

The third controversy. Missing letters in the inscription of the Great King Gwanggaeto Tombstone
In particular, there are five worn-out letters that were supposed to be about the Year of the Hare in 391 in the inscription.
"百殘新羅舊是屬民由來朝貢而倭以辛卯年來□□□百殘□□新羅以臣民"
When Japanese scholars tried to translate the missing part, they suggested a liberal translation that says, "Japan crossed the sea, crushed Baekje, Imna and Silla and subjugated their people." However, the inscription of the Great King Gwanggaeto Tombstone was inscribed to record history of Goguryeo. Therefore, the Japanese supposition of the missing part to be about Japan is not plausible.

The fourth controversy. Gaya people who moved to Japan
According to archeological evidence and records thus-far unearthed, it is presumed that Gaya was not a tributary to Japan but actually it was Japan that might have been part of the Gaya confederacy or that might have broken off from Gaya before the 4th century. Japan's population growth, in particular, was explosive around the 4th century. Compared to the average population growth of a typical agrarian society of the days, it would be impossible to explain such explosive and even mysterious population growth without factoring in the possibility of influx of people from other regions.

The layout of the ancient Japan's founding myth is the same as that of Gaya, and many remains of the ancient Japan were excavated in Gaya. The background of the ancient Japanese dynasty named Gujihurubong also sounds similar to Gujibong of Gimhae in Korea. Above all, Japan was building its national foundations as it received iron making skills of Gaya. Even though there are diverse theories thrown around, the possibility of Gaya people moving to Japan is still valid