Every word of God is pure: he is a shield unto them that put their trust in him. — KJV, Proverbs 30:5
Another gospel is any other gospel than the gospel.
Another gospel is for example water baptism.
 I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
 As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.— KJV, Galatians 1:6-9
Another sign of that people have another gospel is that they promote a false salvation.
How should we deal with people that have another gospel?
So we let them that come with any other gospel than the gospel, we let them be accursed.
We let them that come with false salvation be accursed.