When you read Galatians 1 you can't escape the fact that there are people in Galatia saying things about God which are wrong, at least as far as the Apostle Paul is concerned, and that he is not best pleased about it. (As an aside, Martin Luther maintains in his commentary that Paul addresses the Galatians here "patiently", "fairly excus[ing] their error", "with motherly affection". One suspects that Luther was measuring Paul against his own standard of harsh address here...)
Clearly what we are dealing with in Galatians is heresy. That is to say, it is an error about God and his gospel which is sufficiently drastic to constitute a desertion of grace and a loss of the gospel. I think that is as helpful a description of heresy as any: it is an error which makes the good news impossible. In the case of the Galatians, who are tempted to think that righteousness comes by the law, Paul ripostes that if this were the case "then Christ died for no purpose". In other words, if things stand as the Galatian heretics think, then the good news of Jesus makes no sense. That is what marks their position out as heresy.
There is, however, error which is not heresy. Unless we are very arrogant, none of us will claim to have a perfect understanding of God and his ways. Implicitly, when we confess this lack, we accept that we are wrong in at least some of the things that we believe about God and what he has done. However, these errors need not be such as make a nonsense or impossibility of the central claim that God in Christ has reconciled the world to himself, not counting our sins against us. We are wrong, but we are not necessarily heretics. The distinction is important, because it allows us to get along together with all our misunderstandings without being in a state of constantly judging and condemning one another. We can have sensible conversations about how we feel our own ideas may perhaps more accurately reflect reality than those of other Christians around us, without thereby anathematising any of those Christians. Sometimes, of course, we must pronounce Paul's anathema - but not over difference of opinion.
The fact that there is heresy and the fact that there is error which is not heresy both rely on the fact that God is real and has really done things. This is obvious in the case of heresy: if God has testified that he has sent his Son into the world and justifies us by faith in him, it is wrong and disastrously wrong to deny that he has done this. If God is real, there can be error which is so serious that it just isn't the real God we're talking about anymore.
But the fact that there can be error which is not heresy also points to the reality of God. If we were just talking about a form of words, we could learn them by rote, and all make sure we were saying exactly the same things. But if we're talking about a really existing God, inevitably we will all have somewhat different perceptions of him. This is true of any really existing person: different things about them strike different observers, and two descriptions of their character, whilst recognisably the same person, have some differences. If heresy is avoided, we can learn from each other's different understandings of God's revelation - and avoid anathematising one another.