The gospel makes Christianity unique among all other religions and philosophical systems of enlightenment, approval, or success because while every other system primarily teaches things to do, only Christianity primarily teaches that the things to do are done.
Outside of Christianity, both religious and irreligious systems aimed at personal success or fulfillment hold out for their adherents a certain goal-heaven, nirvana, reincarnation, enlightenment, happiness, and so forth-and then list out a set of steps or instructions to reach that goal. In religious systems, the steps are steps of obedience, commandments that must be followed to reach the stated goal. Christianity has steps of obedience too, of course. Christianity teaching that the God of the Bible is holy and just and that his commandments are meant to be obeyed. But only Christianity teaches that the human ability to obey God's commandments in a way that would merit salvation simply doesn't exist. We can't do it.
Other religions may talk about God forgiving us for our sins, but the way to receive that forgiveness entails a certain level of obedience, as well. We must earn forgiveness. Christianity, rightly understood, teaches that we cannot earn forgiveness. We aren't good enough. Even our best deeds are tainted by wrong motivations, and even our best intentions result in imperfect obedience...The gospel is the good news that our loving God, in a act of sheer grace, sent his Son Jesus to live the sinless life we should've lived and accept as punishment for sin the death we would've died. Jesus died on the cross as a perfect, willing sacrifice in our place...The gospel is the announcement...that God saves sinners by his grace given in the completed work of Jesus, totally apart from the sinners' work of disobedience.
Jared C Wilson, Unparalleled: How Christianity's Uniqueness Makes It Compelling