Great questions, James!

The simple answer is that the Gospel of the Kingdom IS the Gospel of forgiveness and repentance from sin.

We cannot enter the Kingdom of Heaven if we're awash in sin and rebellion.

This is why the message is often: "Repent! For the Kingdom of Heaven is near." Repentance is necessary to enter the Kingdom. Sin must be killed to enter the Kingdom.

Sin that isn't killed will keep you from it.

The Kingdom of Heaven is at hand, and that's why the forgiveness of sins is good news: it means we can enter this Kingdom, delivered from the sins that kept us out.