In my opinion, some scholars have been too hard on Gertrude. There is no textual evidence, after all, that she truly was in on the plot to kill her former husband, King Hamlet. She seems genuinely shocked in Act 3.4 when Hamlet rails:
Almost as bad, good mother,As kill a king and marry with his brother.
And she, in bewilderment replies:
As kill a king?
She has been manipulated by Claudius, who plays on her love for her son, first convincing her to not let him return to college in Wittenberg for reason of his own good, then to allow him to be sent away to England for the same reason (and again, there is no textual evidence that she has any knowledge at all of Claudius's intent to have her son murdered by the King of England.)
And of course, she is manipulated when dies by her husband's own hand when she drinks of the poisoned goblet. Claudius certainly has time and opportunity to stop his wife before she sips the deadly drink, but does not. He manipulatively allows her to go ahead rather than see his own revenge plot foiled. Gertrude is a pawn in a scheme, no master plotter. She is used, manipulated, by Claudius from beginning to end.