You do understand that, at the time that the apostles were preaching the gospel, that "the scriptures" meant the Old Testament only, and that their source for "the gospel" was oral tradition, don't you?
First of all the verse says nothing at all about scripture. Please read the verse again.
Galatians 1:8 But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed!
Now, please prove that the apostles taught (preached) the assumption of Mary and the requirement to believe it. If you cannot prove that the apostles taught it then we must consider the Catholic Church accursed by God.
Catholics like to trot out this "tradition" as if it magically stupefies people. Yet they can't show us that what the Catholic Church calls tradition is exactly what the apostles meant when they wrote the word "tradition". Nor can they show us that the two are the same thing.
We have the above words of Paul to warn us of those who want to make up stuff and claim it to be from the apostles. Then we have his words in the following verse.
1 Corinthians 4:6 Now, brothers and sisters, I have applied these things to myself and Apollos for your benefit, so that you may learn from us the meaning of the saying, "Do not go beyond what is written." Then you will not be puffed up in being a follower of one of us over against the other.
Now, the Holy Spirit inspired Paul to write those words. Do you want to try to say that the Holy Spirit didn't know what was going to be written and preserved for us today? Are you trying to tell me that the Holy Spirit didn't understand that what the apostles wrote would be considered scripture?
Do you expect us to take the words of some man or men over what the Holy Spirit inspired the apostles to write? How does that make Catholics any different then Mormons, Muslims, JWs, or any other religion that relies on some writer other than the apostles? Either prove that what the Catholic Church calls tradition is exactly the same as what the apostles called tradition and that it was written down or expect us to consider whatever the Catholic Church teaches to be accursed just as the Holy Spirit through Paul told us to do.