GND policies differ from school to school. Some of them are school initiated, and others are student initiated. The student initiated variety is basically a student resolution not to disclose grades. The school variety is a policy where schools restrict recruiters that come on campus from asking for grades. This might keep some recruiters away, depending on how much clout the school has and how much interest recruiters have in those students.
For either school or student initiated GND, I think your mileage will vary. Darden has school initiated GND; this was actually not a factor for me so I didn't know about it, but a 2nd year told me that recruiters can't ask for grades here. However, he also said that when you get to the interview room, recruiters will basically say something like "We know that we're not allowed to ask for your grades, but we do need you to give us some kind of indication as to how you did in your first year." It's up to the student how to respond to this, but the 2nd year told me that he always just told the recruiter his results; he also suggested that blowing off this question or refusing to answer might hurt your chances in recruitment. You could probably also point out any honors, dean's list, scholarships or other selective items instead if they are tied to grades. Might be pure speculation. He was interviewing with investment banks.
So, I think the point is that even if GND is in place, it doesn't necessarily mean that your grades will not come up. I do think that school clout and ongoing relations between school & firm will be factors. If a firm has had success over the years with people from your school, then grades will probably matter less. But pure, straight, flat-out non-disclosure? I think that maybe 2 schools can pull that off if they really wanted it. Somehow it always comes down to Harvard and Stanford doesn't it? Wharton has been going back and forth over the last few years, but it's clear that they are feeling the pressure created by recruiters that want to see grades.