anyone who is hung (literally a man hung): The ancient Greek version reads for everyone who is hanged on a tree. Compare Galatians 3:13, which explains how this saying applies to Jesus.
is under God's curse: Sinners who refuse to repent experience God's judgment. But this saying probably means that people would assume God cursed someone in this condition. In reality, the person might have been misjudged or even killed without a legal trial.