Gopas are a tribe or caste of Bombay, India (now Mumbai) who were kinsmen and fighters for Vasudeva Krishna. They were allied with Kauravas, due to a pact made between Vasudeva Krishna, Arjuna and Duryodhana.
The pact of Vasudeva Krishna to Arjuna and Duryodhana:- There is a large body of cowherds known as the Narayanas, all of whom are able to fight in the thick of battle. These soldiers shall be sent to one of you and I alone, resolved not to fight on the field, and laying down my weapons, will go to the other. Arjuna selected Vasudeva Krishna and Duryodhana selected Krishna's army. (5,7)
Many thousands of Gopas with a large Gopayana force battled in the Kurukshetra War, at Kauravas side, against the Pandavas, along with other armies, like the Madras, the Sauviras, the Gandharas, the Trigartas and the Kalingas. (6,71)
Lord Krishna was a Gopa. Amarakosha gives Ballava as well as Abhira as synonym for Gopa.
In Harivansa Purana, it has been said that Gope and Yadav are generic of the same lineage.