In Act 5, scene 2 of Hamlet, does Gertrude know that the wine is poisoned and drink it to save her son's life?
This is a great question! There is no specific textual evidence to definitively say yes or no, but it makes for an interesting discussion of the subtext of the play and what a director can do with looks, actions, timing, etc. to convey her knowledge or lack thereof.
Based on the actual text of the play there is no direct statement that she knows anything is amiss with the cup of wine. For the king to have the cups out and to toast to his son would be standard form for the day, and his putting the "union" -- a very large pearl which he claims 4 kings before him wore in the crown -- in the cup of wine would have been a typical way to pass along the additional reward. It is kind of like a man putting the diamond engagement ring in the glass of champagne at the proposal. These actions aren't all that suspicious. Hamlet turns down the initial offer of a drink, and after the second touch by Hamlet it looks like Hamlet is going to win the fencing match. At this point, Hamlet has been working hard and looks thirsty and sweaty. Gertrude notices this and takes the cup and her napkin (handkerchief) to Hamlet. The sequence of the lines really matters here: first she says "The queen carouses to thy fortune, Hamlet." This means that she is lifting the cup in a toast to him. He acknowledges the toast with saying, "Good madam!" The next line is Claudius's; he says, "Gertrude do not drink." This implies that she is just about ready to take a drink herself and she makes a sharp, potentially brash reply when she responds, "I will, my lord; I pray you pardon me." This line could be delivered sarcastically, implying that she is enjoying her willful defiance of the Claudius's command, thus showing Hamlet that she is doing what he wants and trying to stay away from or stand up to Claudius. No matter how the line is delivered however, the next implied action of the play is that she offers the cup to Hamlet who tells her, "I dare not drink yet, madam; by-and-by." If she had known that the cup was poisoned, I don't think she would have risked offering it to him if there was any chance he would have taken it from her.
Once she starts to feel the effects of the poison her dying words are a warning directly to Hamlet that the cup was poisoned.
Now to the supposition that perhaps she knew. Could she have been suspicious of Claudius's suggestion of the duel and his glowing praise for Hamlet's fencing abilities? Sure. Could she have noticed that Claudius drank from the cup and then put the union in? Sure. Could she have found it odd or suspicious that Claudius warned her not to drink for the cup, so she did so that Hamlet wouldn't? Sure. A director of the play can only have those ideas played out with facial expressions on the queen's face, and perhaps the pace and timing of the delivery of the lines. Shakespeare never wrote explicit stage directions, so the reader has to read the intent of the lines with great care.