There are two kinds of Hateful Gladiator gear, and this is where it can get a bit confusing. There are Hateful Gladiator itemsets and Hateful Gladiator non-set pieces which include bracers, belts, boots, cloaks, amulets, and rings. The non-set pieces are epic and only cost Honor and do not require any Arena points or ratings. These non-set pieces can be purchased from the Hall of Legends in Orgrimmar or the Champion's Hall in Stormwind. There are no Savage Gladiator non-set pieces.
The Hateful Gladiator itemsets, on the other hand, cost a combination of Honor and Arena points, as well as require moderately successful personal and team ratings. These are available only from the Arena Vendor in Dalaran's Underbelly, where the goblins hold their little fight club. However, just like the Savage Gladiator items, there's a way for players to get a complete set of Hateful Gladiator gear without having to set foot in a Battleground or Arena. All Hateful Gladiatior itemset pieces are available for purchase with Emblems of Valor, the token drops from 25-man raids.
Finally, there's a method by which players can acquire Hateful Gladiator gear without spending any Honor, Arena Points, or Emblems of Valor. It takes no more than maybe fifteen minutes, and hopefully a little luck. The normal, or 10-man version of Archavon the Stone Watcher drops both Tier 7 (10-man) as well as Hateful Gladiator set pieces. However, Archavon is not known to drop the head or shoulder pieces. You'll have to pick these up yourself with some PvP or grinding raids.
Sources for non-set items:
Sergeant Thunderhorn in the Hall of Legends, sells for Honor (Horde)
Captain Dirgehammer in the Champion's Hall, sells for Honor (Alliance)
Sources for itemsets:
Magister Brasael in Sunreaver Sanctuary in Dalaran, sells for Emblems of Valor (Horde)
Arcanist Adurin in the Silver Enclave in Dalarn, sells for Emblems of Valor (Alliance)
Xazi Smolderpipe in Dalaran's Underbelly, sells for Honor and Arena points with ratings requirement (Neutral)
Archavon the Stone Watcher in normal difficulty, drops chest, hands, and leg pieces