The fascinating, even spellbinding teaching of some people in the Galatian churches had turned the Galatian believers away from the true gospel. Paul boldly asserts that the different gospel which is so attractive to the Galatian Christians is really no gospel at all. It is a perversion of the gospel of Christ, perpetrated by some people who are trying to cause confusion in the Galatian churches.
Probably these people claimed that their message supplemented and completed Paul's message. They would not have viewed their version of the gospel as heretical. After all, they did not deny the deity of Christ, the cross of Christ or the resurrection of Christ. They subtracted nothing from Paul's message. They only added to it.
But Paul does not allow their gospel to stand as a legitimate option. He sets forth a radical antithesis. His gospel cannot be served alongside other gospels, buffet-style. There is only one true gospel of Christ. The rest of his letter defines the true gospel in antithesis to the false gospel, so that the readers will reject the false and embrace the true.