- I can't find any of my old gladiator mount
- I am missing a gladiator mount from a previous season
Since Battle for Azeroth, the Gladiator mount for the season is awarded as soon as you earn the Gladiator achievement for that season, which requires to win 50 games at 2400 rating or above.
When you earn your achievement, the Gladiator mount is immediately added to your collection.
Since Shadowlands, all Gladiator mounts are account-wide. If you earned a Gladiator mount previous to Shadowlands launch, log in to the character that earned the mount to make the mount available to all your characters on the same faction.
Since mounts are now account-wide, if you transfer your character to a different account, mounts you earned with the character will not transfer, will remain on the original account. This is intended.