Gertrude’s reaction advance the plot of the play BY SUGGESTING THAT OPHELIA'S MADNESS IS A SIGN OF GREATER TROUBLES.
In fact, before the end of Act IV of Hamlet, Ophelia did meet great trouble. She drowned and died. She fell into the river but kept on singing instead of shouting for help. Her clothes grew heavy with water and it pulled her down the river and caused her to drown.