"So with many other exhortations he preached the gospel to the people (Luke 3:18)."
Gospel means "good news." What is curious is John the Baptist's preaching on the surface sounds like anything but good news. Luke's summary of John's preaching contains repeated references to the wrath of God, which sounds like bad news.
"So he began saying to the crowds who were going out to be baptized by him, "You brood of vipers, who warned you to flee from the wrath to come (Luke 3:7)?"
"Indeed the axe is already laid at the root of the trees; so every tree that does not bear good fruit is cut down and thrown into the fire (Luke 3:9)."
"His winnowing fork is in His hand to thoroughly clear His threshing floor, and to gather the wheat into His barn; but He will burn up the chaff with unquenchable fire (Luke 3:17)."
Why does Luke characterize John's preaching as "good news?" Keep in mind that these were warnings, not mere predictions. John was telling the people that judgment and wrath were coming, but there was a way to be saved. That was the good news.
Often times we do not like to talk about judgment and wrath. But not talking about it doesn't do anything to lessen the reality of it all. John warned of it here. Jesus does so many times (Matt 7:19; Mark 9:44; ). Paul does (2 Thess 1:8); Peter does (2 Pet 3:7). Even Paul's sermon on Mars hill to those unfamiliar with the true God concludes with a warning:
"...God is now declaring to men that all people everywhere should repent, because He has fixed a day in which He will judge the world in righteousness through a Man whom He has appointed, having furnished proof to all men by raising Him from the dead (Acts 17:30-31)."
I could go on and on, but you get the picture. There are a lot of warnings in nearly every book of the Bible about God's judgment and wrath. It is not pleasant. It is not something we want to dwell on. It is not something we need to dwell on. We have been taken out of the kingdom of darkness and transferred into the kingdom of light. However, not dwelling on it does not mean we should ignore it.
It is good to talk about the proofs that God has provided, such as raising Jesus from the dead. It is good to discuss how Jesus fulfilled the Old Testament promises. We should share who Jesus is and what he accomplished at the cross and tomb. But do not avoid the messages of sin and judgment. Hell is real every bit as much as death is real. Ignoring it will not make it go away. Teaching about it doesn't lesson the goodness of the good news in any way. If anything it strengthens it. Bringing it to the attention of your loved ones doesn't make you mean. It may be the most compassionate thing you can do. After all, we do want them to be saved. It also can wake sleepy people up to the urgency of the gospel.