It's a fairly lengthy walk. GIG-ATL will come into Terminal F, she'll have to clear customs and immigration, re-clear security, hop the train most likely to Terminal B, and probably have to walk half way if not all the way down to the end.

Looking at schedules, there are often flights getting in from LOS, EZE, UIQ, BSB, SCL, JNB, and DXB all around the same hour as the GIG flight. I've never personally been through at that hour of the morning, but perhaps someone who has can speak to what that means for the lines at customs and immigration. Keep in mind the potential for long times standing in line in addition to just the walk. I've often seen pax in wheelchairs escorted straight through without having to wait in line.