To whom is Gertrude speaking when she advises him, "More matter, with less art"?

Polonius.

Explanation:

William Shakespeare's tragic play "Hamlet" tells the story of how a murdered king's son took revenge n his own uncle and mother for their crimes. But that also involves the fateful death of his own life, for the sake of the revenge.

In Act II scene ii, we see the King Claudius and Queen Gertrude in conversation with their trusted friend Polonius about the young prince Hamlet's condition. Hamlet have seemed to be insane, acting rationally against his mother and uncle/step father. When Polonius had suggested that he had found out the reason for the condition of the prince, the Queen advised him to state the facts and not beat round the bush. This quote/line "More matter, with less art" is thus addressed to Polonius.