Kalgan had the following to say about Arena Gladiator set bonus changes:
As of patch 2.3 all items in the gladiator season 1, 2, and 3 sets will be part of the same set, so you can use those items interchangeably and keep the set bonuses (ie: you can wear the season 2 shoulders along with the season 3 helm, chest, legs) and retain the 4 piece set bonus. Many of the set bonuses have been updated. The new 4 piece set bonuses are also retroactive as a result of the above change.
No longer will you be able to stack two +35 resilience bonuses from the Gladiator sets, but hopefully the new 4-piece set bonuses will be compelling enough that you wouldn't want to.
Read the original thread here.
http://forums.worldofwarcraft.com/threa ... 1&pageNo=3