StandFirm has kindly affirmed my position:
What Galatians 1:8 and other such scriptures are speaking about is the overall message that someone might be preaching. If an individual's or group's overall message is different than the one the Christians of the 1st century preached, then they fall under that condemnation.2 Cor 5 details what the 1st century gospel message was about:
Jehovah's Witnesses are trained, by the Governing Body (the group of men based in North America who claim, without corroborative evidence, to be God's spokesgroup), to preach a different gospel. And they don't even try and hide it:
18 But all things are from God, who reconciled us to himself through Christ and gave us the ministry of the reconciliation, 19 namely, that God was by means of Christ reconciling a world to himself, not reckoning to them their trespasses, and he committed the word of the reconciliation to us.
"Let the honest-hearted person compare the kind of preaching of the gospel of the Kingdom done by the religious systems of Christendom during all the centuries with that done by Jehovah's Witnesses since the end of World War 1 in 1918. They are not one and the same kind. That of Jehovah's Witnesses is really the "gospel" or "good news," as of God's heavenly kingdom that was established by the enthronement of his Son Jesus Christ at the end of the Gentile Times in 1914." Watchtower May 1st 1981 page 17Where in the 1st century gospel was any mention of 1914?
Therefore, per StandFirm's assertion, the Witness' overall message is different from the message preached Paul, for example.