I am astonished that you are so quickly deserting the one who called you by the grace of Christ and are turning to a different gospel – (NIV)

I am surprised and astonished that you are so quickly turning renegade and deserting Him Who invited and called you by the grace (unmerited favor) of Christ (the Messiah) [and that you are transferring your allegiance] to a different [even an opposition] gospel. (AMP)

I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel: (KJV)