In Galatians chapter one, Paul marvels that the Galatians are "so soon removed" to "another gospel." Then he goes on to say:
Galatians 1:7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
Notice that he says "which IS not another," not "which WAS not another," nor "which WILL NOT BE another." The whole point being, that IN TIME PAST there was another gospel being preached which is not the same as Paul's gospel. The point being that IN THE FUTURE there will be another gospel preached which is not the same as Paul's gospel. Now you can clearly see that from the next verse:
Galatians 1:8 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.
All you have to do is compare Revelation 14:6-7 with 1 Corinthians 15:1-4 and you can easily see that the angel from heaven in the book of Revelation is not preaching the same gospel that Paul preaches in 1 Corinthians 15. Paul preaches a salvation message but the angel in the book of Revelation preaches a message of judgment.
The issue that Paul is presenting is that some are perverting the gospel of Christ that he preached. How did they do that? By adding works to the grace message. In time past, when another gospel was being preached, works PLUS faith were required. Peter even said that men who feared God AND worked righteousness are accepted with him.
But Paul said Christ died for sinners, for the ungodly, and even for the enemies of God. Paul said that God was in Christ, reconciling the world unto Himself, NOT imputing their trespasses unto them. Paul said "believe on the Lord Jesus Christ, PLUS NOTHING, and thou shalt be saved."
The issue in Galatians is not a FALSE gospel as so many claim today, but rather a DIFFERENT gospel, which was a TIME PAST message. In Romans 11:6 Paul is clear that salvation is NO MORE of works. In the past men had to DO something to be saved, today they have to BELIEVE something.