# I am amazed

"I am surprised" or "I am shocked." 

# you are turning away so quickly from him

"you are so quickly starting to doubt him".  Here "turning away ... from him" means to doubt or to stop trusting God. 

# him who called you

"God, who called you." This means God has appointed or chosen people to be his children, to serve him, and to proclaim his message of salvation through Jesus.

# by the grace of Christ

"because of Christ's grace" or "because of Christ's gracious sacrifice"

# you are turning to a different gospel

"you are instead starting to believe a different gospel"