Guest Relations Locations:
AK: Outside the park: To the left of the main row of turnstyles, if you're facing the turnstyles.
Inside the park: To the right of the main row of turnstyles, if you're facing the turnstyles.
Epcot: Outside the park: International Gateway
Outside the park: To the right of the main row of turnstyles, if you're facing the turnstyles
Inside the park: "Under Mickey's Elbow" (roughly)
MK: Outside the park: To the right of the main row of turnstyles, if facing the turnstyles
Inside the park: City Hall ... go under the train station, it's on the left as you're facing the castle, before you get to Main Street.
MGM: Outside the park: Facing the main row of turnstyles, turn left. Guest Relations is between the gift shop and the Kennel
Inside the park: To the right of the main row of turnstyles, if facing the turnstyles.
DTD: two locations, I'm not exactly sure where either one of them is, I'm also not sure whether or not DTD Guest Relations can issue GAC's
All of the locations (with the possible exception of the 2 DTD locations) can issue a GAC. This being said, a lot of times, the folks outside the park will be rather busy with ticket issues, and it might be easier to go to the inside GR location for GAC questions.