The main difference between Rooster and Gamecock is that the Rooster is a male chicken and Gamecock is a blood sport.
A rooster, also known as a gamecock, cockerel or cock, is an adult male gallinaceous bird, usually a male chicken (Gallus gallus domesticus). Mature male chickens less than one year old are called cockerels. The term "rooster" originates in the United States, and the term is widely used throughout North America, as well as Australia and New Zealand. The older terms "cock" or "cockerel", the latter denoting a young cock, are used in the United Kingdom and Ireland."Roosting" is the action of perching aloft to sleep at day, which is done by both sexes. The rooster is polygamous, but cannot guard several nests of eggs at once. He guards the general area where his hens are nesting, and attacks other roosters that enter his territory. During the daytime, a rooster often sits on a high perch, usually 0.9 to 1.5 m (3 to 5 feet) off the ground, to serve as a lookout for his group (hence the term "rooster"). He sounds a distinctive alarm call if predators are nearby and will frequently crow to assert his territory.
A cockfight is a blood sport between two cocks, or gamecocks, held in a ring called a cockpit. The history of raising fowl for fighting goes back 6,000 years. The first documented use of the word gamecock, denoting use of the cock as to a "game", a sport, pastime or entertainment, was recorded in 1634, after the term "cock of the game" used by George Wilson, in the earliest known book on the sport of cockfighting in The Commendation of Cocks and Cock Fighting in 1607. But it was during Magellan's voyage of discovery of the Philippines in 1521 when modern cockfighting was first witnessed and documented by Antonio Pigafetta, Magellan's chronicler, in the kingdom of Taytay. The combatants, referred to as gamecocks (not to be confused with game birds), are specially bred and conditioned for increased stamina and strength. Male and female chickens of such a breed are referred to as game fowl. Cocks possess congenital aggression toward all males of the same species. Wagers are often made on the outcome of the match. Cockfighting is a blood sport due in some part to the physical trauma the cocks inflict on each other, which is sometimes increased by attaching metal spurs to the cocks' natural spurs. While not all fights are to the death, the cocks may endure significant physical trauma. In some areas around the world, cockfighting is still practiced as a mainstream event; in some countries it is regulated by law, or forbidden outright. Advocates of the "age old sport" often list cultural and religious relevance as reasons for perpetuation of cockfighting as a sport.
roost + -er
rooster (plural roosters)
game + cock
gamecock (plural gamecocks)