What does "AUX" mean?
The name "AUX" is a nod to the origin of our version of GNAT. Some guys decided they liked Ada so much that they wanted to create an Operating System with as many userland components as possible rewritten in the Ada language. This system was called AuroraUX, but it didn't get very far.
The major components of the system were given codenames based on constellations and other stellar phenomenon, such as Hydra, Nebula, Singularity, Supernova, and Universe. The first version of AuroraUX was based on an OpenSolaris kernel, but before Oracle purchased Sun Microsystems, the project decided to switch to DragonFly BSD. One problem: DragonFly BSD didn't have an Ada compiler ported over to it yet.
John Marino didn't want to join the AuroraUX effort, but he was interested in having an Ada compiler available for DragonFly, and after quite a bit of effort and experimentation, he was able to provide compilers for both architectures that DragonFly BSD supports: ix86 and x86_64.
The AuroraUX project members took to referring to the project as "AUX" or "aux" rather than typing out the full name, so that's why this version of the GNAT compiler is called GNAT AUX. Despite the AuroraUX project stagnanting, this part of it lives on independently.