If you gloat, you express great satisfaction at the misfortune of others. If your team scores a big win, it would be better not to gloat. Be happy for your win, but don't laugh at the other team's loss.
Gloat seems have come from a Germanic word meaning "to stare." Somehow people may have gotten the sense that whoever was doing the staring was also taking a deep and unwholesome pleasure in whatever he or she was seeing. Think about people staring and pointing and laughing. Sometimes you gloat when someone has rejected the help you have offered. If your little sister won't listen to your advice on how to improve her essay and then comes home with a bad grade, it will be very hard not to gloat and say "I told you so."