I just wanted to add that I get the option to customize the gate at Imaginesim's KATL but no afcad is shown as selected when the tool launches. I am using an afcad I created for the airport. AES is enabled. It seems that somehow the tool can't find the afcad but otherwise GSX still performs its normal functions.
The short term solution is to either 1) use GSX at KATL and ignore the second stair vehicle through the jetway; or, 2) refrain from using GSX at KATL.