Stanford GSB and HBS

I think GSB has an acceptance rate of 7% and HBS has a rate of 11%. If you look at the undergraduate acceptance rates to the two institutions, it hovers at 5-6%. Given that undergraduate admissions are heavily a crapshoot and weighted heavily on non-academic, extracurricular pursuits given the ubiquitous nature of 4.0, 2300+ SAT applicants, how difficult and predictable is GBS and HBS admission compared to the volatility of undegrad admission?

Will any of Duke, Dartmouth, or Brown be more favorable in gaining admission to this caliber of institutions? If a good GPA and high GMAT is obtained, does that almost guarantee admission?