The Gospel of Wealth was a philosophy that argued that people who had huge amounts of money should spend the money in ways that would help their society as a whole. This idea was put forth by Andrew Carnegie as a way (in part) to defend the rich and justify their wealth.
We can see examples of this sort of attitude among the rich today. There are many rich people who have set up foundations or other organizations to give money to certain worthy causes or to help people in ways that the philanthropists think important. Perhaps the most famous example of this is the Bill and Melinda Gates Foundation. The Gates Foundation is well known for giving money to improve education in the US and around the world and also for spending money to try to improve the health of people in poorer countries. The Gates's actions would be an example of the Gospel of Wealth.