It’s currently not possible to complete the current-season’s gladiator PvP armor set for shaman.
The current PvP gladiator cloak, the Theurgic Starspeaker’s Shawl, is not available for purchase like the rest of the PvP gear from Zo’sorg in Oribos. Only four cloaks are available from Zo’sorg, none of which correspond visually to the shaman’s PvP cloak.
Oddly, this missing cloak is also named differently from every other piece of the PvP set, which is “Cosmic Gladiator.” If you log in with a shaman and view the set on the Collections tab, you’ll see that every piece is named Cosmic Gladiator… except for the cloak. I suspect the PvP cloak is incorrectly named in addition to be unavailable. (The PvP bracers had a similar naming problem until the most recent Tuesday patch, but have always been available for purchase).