kitchkat,
something seems not right here, but maybe I'm misunderstanding...do you have drawings with specs? was a template made for the granite? Did you speak to the appliance folks and look at the specs?

In fact the wolf spec design guide--you should get one from your appliance seller or I believe it's actually online--states for a 36" gas cooktop a 39"cabinet width is RECOMMENDED altho the minimum is 36" AND the cooktop cut out width is 35". I am not a designer nor spec expert but to my mind the granite fab'rs need to know the cut-out specs--the depth cut out should be 19 1/4 " per the spec book.
Maybe your designer has had problems w/ a 36 inch cab under this cooktop or maybe it looks funny. Hard to know but you should really feel sure you are getting what you want or understand why want you what will not work.

Personally I would stop everything until I had a better understanding, but then maybe I'm not understanding the whole picture--good luck!