The Gatekeeper fight seems to have been changed a bit. At the start you will get a buff called "Heal Suppressor" that cannot be cleansed or cleared. It suppresses all heals from active abilities, passive abilities, leeching and even gadgets. The only thing that works is healing potions.
This means you should ignore advice about heals like Immortal Warrior, Forged in Fire, or Leeching mentioned in older guides. Instead focus on abilities that boost your block, evasion and protection. Apart form that the old guides are still valid: Dodge the column attack and impair Gaia Core. Quickly get aggro from the adds when they spawn (you don't need to kill them, they do very little damage to you) to protect your green friend. The Gatekeeper will knock you down and reset aggro at one point so be prepared to get it back as fast as you can (rocket jump, flamethrower or mass provocation works fine).
You will probably want to drink a barrier potion at the start to prevent some damage and then a healing potion when 90 seconds have passed. This is the build I used to pass the fight:
- Grip Whip to attack Gatekeeper (boosts block)
- Escalation (to draw aggro from adds when they spawn)
- Dragon's Breath (to aggro adds after being knocked down)
- Illusion (boosts evasion)
- Chaotic Pull (impair)
- Helter Skelter (impair)
- Hardcase (boosts protection)
- Cool, Calm & Collected (reduces damage after successful block)
- Attack rating: 99
- Health: 10204
- Block rating: 368
- Evade rating: 91