First off, I'm still not convinced he's dead.

Second, Gaius' primary method of operation was subterfuge and espionage.

Third, Gaius acted on his own due to the Civil war that followed Emperor Soluz's death.

Lastly, The Battle of Silver Tear Skies was the VIIth Legion not the XiVth. Neal's VIIth Legion was the "Strike Team" that lead frontal assaults where as Gaius was used to hold captured territory (ie Ala Mihgo).

As for his age, I always assumed mid to late forties since Cid looks to be in his Early 30s.