Believing the Gospel IS repentance

The word repentance simply means to change your mind. Under the Gospel of the Circumcision it technically can include a change of actions as well, but under Paul’s Gospel it simply means that you’ve changed your mind about being able to save yourself in any way, and realized that only what Christ accomplished can save you.

Basically, when you truly believe the Good News, it means you’ve repented of your previous mindset that you could save yourself through works (even the work of having to choose to believe something specific, or having to choose to have faith, which is probably the hardest work one could possibly do) because God has now given you the faith to believe that Christ already took care of it for everyone through His death for our sins, and His subsequent entombment and resurrection.