Some caster can be fully decked out in PvP gear and have a huge GS but pull less DPS than someone in a proper full Heroic gearset because of the Spell Hit cap. Or someone can be itemized completely the wrong way (like an Affliction Warlock or Combat Rogue gearing for crit). And there are a fair bit of items with lower GS that are better than items with a higher GS. TBC is the expansion with the most lower level items being better than higher level items. GS is deeply flawed in TBC, because TBC really isn't about bigger number = better item. As a Rogue for example, before I quit because the GS cancer was too much on this server, I even outDPSed Hunters and Warlocks with a higher GS than me because I itemized properly (i.e., 2 pieces of Wastewalker for the hit rating, low GS though because they're blue items!, still BIS in T4).
Not to mention that any complete scrub can buy donation gear to pump his GS but have absolutely no clue on how to play his class. I'd rather take the full Heroic geared player because you know he didn't buy his gear (why would you buy blues when you can buy epics?) and actually knows how to play his class.
GS could be useful if it's used as a very rough indication but that's not the way it's used on Outland. On Outland it's used to cut off anyone with a lower number. The amount of heroics and pugs I wasn't invited to because my GS was "too low", even though I usually topped the meters and was always in the top 5... GS is cancer.