Your original question contained more than one question. Please remember that enotes does not allow you to ask multiple questions. I have edited your question accordingly.
Act II scene 3 and Act III scene 1 are identified as the "gulling" scenes as you mentioned because of the "gulling" or tricking that goes on. In these scenes, in a masterful use of deception and appearance, both Benedick and Beatrice are made to believe that they are loved by the other. This is clearly a hilarious plot device by Shakespeare given the mutual hate and conflict that has characterised their relationship up until this point. However, if you read the play carefully, it is in Act II scene 1 where Don Pedro comes up with this scheme and asks the assistance of his friends:
I will in the interim undertake one of Hercules' labours, which is to bring Signor Benedick and the Lady Beatrice into a mountain of affection th'one with th'other. I would fain have it as match, and I doubt not but to fashion it, if you three will but minister such assistance as I shall give you direction.
Thus it is that Don Pedro enlists the help of Claudio and Leonato to help him trick Benedick in Act II scene 3, and Hero and Ursula, to trick Beatrice in Act III scene 1.