The magicians tried by their secret arts to produce gnats, but they could not. So there were gnats on man and beast.
They "did" means they tried, not that they succeeded; the text says this plainly when it adds they could not. God removed Pharaoh's last excuse to hide behind the magicians' tricks, and stripped these imposters bare before all Egypt, that He alone might claim the glory of power without rival.
AI summary
The dust itself obeys God's command; He has infinite arrows in His quiver. When the magicians failed here, they had to confess "This is the finger of God", a divine power that has the devil on a chain, and stops him short with nothing but a gesture.
AI summary
Commenting on Exodus 8:16-19
So long as the magicians could counterfeit or deceive the eye, they denied God's power; but when they could no longer produce these gnats, they broke and named a superior force. Yet even then they called it Elohim, the Supreme Deity, not the God of Israel, whom they took for a petty thing, and Pharaoh's heart hardened still.
AI summary