Loki, you can find information about the GoC = Gem of Completion on the Wiki... it's under quest "The Mage Inquisition Quest".
Basically you collect some 'special' items from the peerless and some mobs in that area, once you collect the items you will receive a gem for that particular area (e.g. sanctuary gem or gem of sanctuary) after you get all 6 gems you will receive the final GoC which you can trade for one of the powerful armour/shield.
Word of advice when you add the last item inside the gem collector (which you get in a room at the Excelsior Hall, just say the dungeon you want to collect the items from... e.g. bedlam, sanctuary, parox....etc...) make sure you have the collector in your bag and if you have any pets out... shrink them! I made the mistake of putting the last item and I was teleported without my pets to the room where you exchange the collector for one of the gems! I had to go back and get my pets afterwards!
Be prepared...be very prepared since I believe these are quite tough mobs that won't relinquish their items so easily... I have already tackle 1 of 6 gems with help of some players and I can say that some are bad and some are VERY bad
If you got more questions feel free to pm me when I am online... cu in game!
Now offering free tours to Sgail and Umbris camp, bring your own weapons as safety is not guaranteed.