To open the 1st gate, none of the Devotees are allowed to die. There should be 3 groups, they are all hostile but shouldn't attack, instead you have to pull the aggro cultists far away from them and then be careful with your AoEs. The Devotees will then turn blue and run away. The already blue ones in front of the gate dont count, if you did everything correctly the gate will already be opened. Do it solo, it's impossible in PUG.
To open the 2nd gate, kill all cultists (except the devotees). There are tons of missable groups in all dead ends and e.g. behind the last group of devotees (this is the most easily missable). If it isn't open when you arrive at it, backtrack and search everything again for groups you missed.