As noted, Billy Gunn was fired from his job as a trainer at the WWE Performance Center after he tested positive for performance enhancing drugs at a power-lifting competition back in July. WWE was not aware of Gunn's suspension or the fact that he was doing these competitions.
Bryan Alvarez noted on Wrestling Observer Live that it's believed WWE only tests their contracted talents, not the coaches and staff at the Performance Center. This is probably why there was never a negative WWE Wellness test for Gunn. However, WWE officials were forced to release Gunn because they can't have one of their trainers testing positive for performance enhancing drugs as it makes their entire developmental program look bad.
No word yet on who will replace Gunn but WWE officials are reportedly considering two people.