The famous gulling scenes, in Act II scene 3 and Act III scene 1 when Benedick and then Beatrice are tricked into believing that the other is in love with them, are key moments in the play and also very funny when we see how they are taken in and transformed by this deception into precisely the kind of love-struck characters that they had previously deplored and ridiculed. However, regarding the setting, I suppose that one of the main reasons that these scenes occur in the garden is so that Benedick and Beatrice can have somewhere to hide behind whilst they, supposedly, "eavesdrop" on the conversations of their friends. However, having it set in a garden also allows the friends to make sure that they know where Beatrice and Benedick are and to ensure that they continue to listen throughout their staged conversation.
Think too of how this would appear to the audience. You could have Beatrice and Benedick hiding behind a tree or a hedge, with the audience able to see both them and the other characters, so that we can see their reactions and how funny they are.