In Galatians 4:21-31, Paul uses the story of Abraham, Sarah, and Hagar as an illustration to help us understand that God's salvation has never come through human effort. Rather, God's salvation always has and always will come through his gracious acts for us. These Galatians were being tempted to produce through salvation through their Law-keeping—through their human effort (like Abraham and Hagar tried to produce the promised child through theirs). But Paul reminds them that in order to be justified before God all we need is to trust God's promise to save in Jesus (like Abraham and Sarah later did, exemplified in Isaac being born to them in their old age). Paul further uses this illustration to remind us that anyone who is trying to earn their own salvation is simply in slavery. But those who trust Jesus are free because through faith everything he did in his life, death, and resurrection counts for them.
In the end, the gospel is bad news for those who want to save themselves because the gospel highlights the fact that we could never be good enough to earn God's blessing—we were so wicked Jesus had to die for us. On the other hand, the gospel is good news for those who know they can't save themselves and trust in Jesus' work in their place. The gospel not only highlights the fact that we were so wicked Jesus had to die—it also highlights the fact that he was willing to die, to take away our sin and make us righteous. As Paul says elsewhere, "The saying is trustworthy and deserving of full acceptance, that Christ Jesus came into the world to save sinners, of whom I am the foremost" (1 Timothy 1:15).