This is of course a very important question when we consider the play as a whole. However, this is also a very subjective question, and I think it may be good to repeat it in the discussion posting section of this group so that you can gain a wider input in your own thinking as you try to decide what your answer to this question is.
Personally, I think throughout the play it is referenced how swift and rash Gertrude's decision to re-marry so quickly was. It has clearly upset Hamlet, but we can infer that others are questioning this move as well. Thus in Act II scene 1 the speech of Claudius is an excellent example of how he confirms his rule and power and attempts to gloss over their rather hasty union. However, Hamlet's first soliloquy in this scene clearly reveals his feelings:
That it should come to this:
But two months dead: nay, not so much; not two,
So excellent a King, that was to this
Hyperion to a satyr: so loving to my mother,
That he might not beteem the winds of heaven
Visit her face to roughly.
As Hamlet concludes, "Frailty, thy name is woman." Nobody can blame Gertrude from wanting to remarry, but the massive change in her affections and emotional state leaves her open to criticism. This is something that Gertrude herself recognises when Hamlet confronts her in her bedchamber after the play and the swift exit of Claudius:
O Hamlet, speak no more.
Thou turn'st mine eyes into my very soul,
And there I see such black and grained spot,
As will not leave their tinct.
This quote shows how, when confronted with the truth of what she has done by her son, Gertrude herself recognises that she has acted hastily and wrongly. Therefore I think Hamlet is perfectly justified in how he is feeling.