Paul marveled at the Galatians' turning away. It was only 20 years before someone perverted the gospel into something that was no longer good news!
In less than two decades, Jesus' message had been perverted to something that no longer announced 'good tidings.' A change in the gospel changes its goal.
In Colossians 2:16 and Galatians 4:9-10, Paul was warning against mixing Gnostic asceticism and pagan customs with the keeping of God's Sabbath and Holy Days.