Who were the Gokturks?
A Gokturk coin, about 576-600 AD (Kyrgyzstan museum)
(compare to Sassanian and Byzantine coins of the same time)
About 550 AD, Bumin Qaghan and his sons, with the help of the Chinese Wei kingdom, conquered the Rouran and took over as the main rulers of Central Asia. Today, historians call Bumin Qaghan's people the Gokturks. As their name suggests, the Gokturks were Turkish people, and they spoke a Turkic language, not Indo-European, though their empire included both Turkic people like the Uighurs and people, and even rulers, of other ethnicities. The Gokturks used their empire and their friendly alliance with China to get control of the Silk Road and the trade between the Sassanians and Wei China, which made them rich.
The Gokturks, under Bumin Qaghan's son Muqan Qaghan, formed an alliance with the Sassanians to their south. The Gokturks used that alliance to make their empire bigger and bigger. This forced the Avars to move west into Europe. The Gokturks followed the Avars west, and did a lot of raiding in Russia in the 500s AD. The Gokturks also raided to the south, taking over Sogdian land from their Sassanian allies. And by the 600s AD, with new people ruling China, the Gokturks made an alliance with the new kingdom of Tibet and the two states began coordinated raiding to the east, into China.
The Sui Dynasty Chinese emperors didn't like the Gokturk and Tibetan raids. When the Gokturks got into a civil war, the Chinese emperors took first one side, then the other, getting the Gokturks to keep fighting each other and getting weaker and weaker. Often some of the Gokturks invaded China.
By the 700s AD, these long civil wars were making the Gokturks weaker and weaker. By 742 AD, the Gokturks were so weak that the Uighurs, with the support of the T'ang Dynasty emperors, were able to overthrow the Gokturks (and push back the Tibetans) and start their own independent Uighur kingdom.