Gospel of Wealth was created by Andrew Carnegie in 1889, he defended that his and other industrialists wealth would eventually help to develop technology and provide jobs. He also claimed that anyone could work hard and become rich. Many opposed to his ideas because people knew that industrialists got richer by making corrupt deals.
Social Darwinism was the philosophy that only the fittest individual survive in a capitalist society.
Social Darwinism is more closely aligned with the traditional American values because it defends that capitalism should adjust itself even if this creates large industrialists. It also puts the success of industries on fate or God. It basically shows the laissez-faire philosophy where the government should not intervene in the economy.