I just want to tell some people out there that the GS number is not based off of your gears ilvl... Yes GS will tell you someone's average ilvl however the actual GS number itself is gotten by adding up the stats from all of your gear into one large arbitrary number.
On Dark Iron - US - Alliance
VoA10 - no GS req
VoA25 - no GS req
ToC10 - 4.5k GS
ToC25 - 4.7k GS
ICC10 - 5k GS
ICC25 - 5.4k GS
from what i am seeing though my realm is more forgiving then a lot. beyond that a lot of the time people still get in if they ignore the GS request and just link the achievement.