"Corporate/M&A" is only part of the picture, especially for a New York practice. It's a headline maker to be sure, but it's only a piece of the picture for major corporate practices.
Nationwide GDC is band 3 for capital markets/debt and equity, band 4 for banking/finance, band 3 for tax (which is actually a big deal in major transactions, particularly for M&A where it can be a driving force), band 3 for private equity, band 4 for banking regulation, and band 3 for fund formation. It appears they don't do derivatives, structured products, or bankruptcy/restructuring to any significant extent nationally at all.
Comparing GDC to the big name corporate players in NYC, you'll see that it holds its weight in M&A but is otherwise only solid in the other categories. And those other categories are a big deal - Cravath is a big deal in the M&A world, but it's still doing a shit ton of capital markets, banking/finance, etc. work and that's what makes its corporate practice so large and so sterling.
Concrete example: At one top NYC firm I am familiar with, M&A lawyers make up a large chunk of the firm but are a minority (probably only 20-30% or so) of the firm's "corporate" attorneys.
Don't get me wrong - all of the Vault ranked big firms are good at what they do, profitable, etc. Bands 2, 3 and 4 of the chambers rankings are filled with good firms that are filled with talented and rich lawyers. But Gibson's corporate practice isn't on the same level as the name-brand V10/V15 NYC firms.
All true. GDC's litigation practices stack up well against any of the name-brand V10/V15 NYC firms, however.