You mean AMT as in "ATA Airlines", formerly known as American Trans Air, which closed in 2008 ? GSX never had a livery for such airline so, it's entirely normal nothing would happen if you add this code.
Yes, American Trans Air
As you can see in the image below (zoom in) you can see I added ATA to the GSE using the GSX paintkit. I expected that adding ATA to jetways and other airlines e.g. SWA, ASA, AAY etc. that are not included in the logo pull down menu would only require editing the relevant files; sim.cfg, etc.
https://imgur.com/pVHCiF9In trying to add ATA I'm following the same method as the texture.FSDT folder.
I have added in the FSDT_SODE_jetway_logo folder (Texture.TZ) that contains the
GSX_logo_4x1.dds file (that I use for the GSE). And also the texture.cfg file again following the same method from the FSDT texture.cfg file.
My Texture.cfg:
[fltsim]
fallback.1=..\..\FSDT_SODE_JETWAY_LOGO\TEXTURE.TZ
fallback.2=..\texture
So the question is how to add additional logos to the
logo pull down menu in order to select that logo.