I regularly arrive at ORD on an AS flights at G4 or G6, the ground agent opens the door, gets on the PA system and always announces something to the effect of: "I have an important announcement, this plane has arrived at terminal 3, your baggage can be reclaimed at terminal 2, please disembark and at the rotunda, go left to T2 baggage claim."

That said, I agree with BearX220 that the terminal designation is appropriate to landside services (check in, baggage claim) but not so important if you have checked in online and do not have any checked baggage. Airside, it's not very important to categorize the G concourse one way or the other as terminal 2 or terminal 3. It's kind of like concourse D at LAS: accessible from both T1 and T3 by trains in opposite direction, and occupied by some T1 airlines and some T3 airlines.