Why do we not need the law if the gospel is empowered by the Holy Spirit?
The law is weak and beggarly from man's standpoint because it has no power to enable obedience. The gospel, however, is empowered by the Holy Spirit, who sets believers in the right direction and takes up where the law leaves off, so they are not under law and do not need it.
The teaching explains that the Galatian believers were turning back to the weak and beggarly things, which shows what the law was from man's point of view. Though the law was perfect, absolute, and righteous from God's standpoint, from man's it was weak and beggarly because it had no power to help people keep its demands. But the gospel is empowered by the person of the Holy Spirit, and He sets us in the right direction. Therefore we do not need the law and are not under it, because the Holy Spirit takes up where the law leaves off. Paul further says going back under legalism is a sign of weakness, such as observing days, months, times, and years; we are not under any particular day, feast day, or celebration day.
But now, after that ye have known God, or rather are known of God, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage?