This wasn't a "recall" (which are usually only for safety reasons) but a Technical Service Bulletin, which is a dealer advisory only. So the dealer is under no obligation to repair the vehicle for free.
I think your complaint is with the dealer, not with GMC therefore. Ask them to look up this TSB and to offer you at least partial relief.
A clogged drain, if that were the problem (this TSB suggests otherwise, but you never know) is something you might be able to clear out by buying a can of compressed air and squirting it in the drain hole (you can see them in the runners of the sunroof).
You can test the drains by pouring a little bit of water in them...if they are working properly, you'll see the water draining out the bottom, by the front or rear wheels. (some roofs have two drains per side).
Here's what the TSB says:
Bulletin No.: 10-08-67-001A
Date: February 26, 2010
Subject: Headliner Wet/Water Leak into Cab from Front Sunroof Glass (Replace Sunroof Window Weatherstrip/Seal)
2008-2009 Buick Enclave
2007-2009 GMC Acadia
2007-2009 Saturn OUTLOOK
This bulletin is being revised to update the Cause and Correction information. Please discard Corporate Bulletin Number 10-08-67-001 (Section 08 - Body and Accessories).
Some customers may comment on a water leak coming from the front sunroof glass, which may result in the headliner being wet and/or water leaking into the cab.
The sunroof glass panel weatherstrip/seal may have failed to retain its designed shape, resulting in a water leak that overwhelms the sunroof module water trough.
The sunroof glass seal is not designed to be completely water tight. Minor drips witnessed dropping into the water management system can be expected. Heavy streams of water passing by the weatherstripshould not be evident.