It is not clear if Paul is referring to what he said to the Galatians on his earlier visit to them Acts 13:2 – 14:28) or that which he said in the previous verse.1 Most probably it has to do with his earlier visit.2 The Galatians heard the true gospel from Paul and he warned them at that time that they must not substitute it for another that includes works/calls for circumcision and obedience to Mosaic law.
9 As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.