This is a DPS build for the gatekeeper i used pistols/elemental build.
1) Big Forty Five (pistol consumer)
2) Gun Crazy (elite)
3) The Business (builder; pistol)
4) Thor's Hammer (elem consumer)
5) Magnetic Wipe (low CD and heavy hitter purge for phase 2)
6) Flicker (teleport for phase 3)
7) Deadly Aim (crit buff)
1) Mad Skills (each crit stacks crit rating)
2) Elemental Precision (10% damage increase to elem)
3) Knuckle Down (resource builders deal 10% more damage)
4) Brawler (crit power increase by 15%)
5) Live Wire (elite; 4 crits create an additional heavy hit)
6) Straight Shooter (10% damage increase to pistols)
7) Calling the Shots (reduces the recharge time of deadly aim by 30s)
I had full blue Q10 gear so high attack power (2455) but low health (1960)
i had health pots ready but did not need them
A description of the phases.
The Gatekeeper himself has 3 phases you must be aware of, I'll talk about them a bit in the following part:
When I started the fight I allowed him to create two AoE circles far away from him (he creates them right under your feet) which allowed me to easily circle around him with having much room. It's handy since he will keep making the AoE circles, but other than the first 4 ones he places down before the fight, they will disappear one after another soon enough. First stage is all about dealing damage and moving a bit here and there, I started with building 5 resources for both elem and pistols and then I popped my critical buff, followed by my elite active. The active hits a lot and will naturally crit much as well, especially with the buff being active, allowing me to max critical rating stacks to 5 very fast and keeping it there mainly with my pistol resource builder (burst fire and whatnot).
After that came the buff phase where I had to use purge, I suggest watching out for his casting animation and making sure you got elem resources ready for Magnetic Wipe at that time. He will buff himself twice and pretty fast, hence the purge must have a low cooldown -- Magnetic Wipe is perfect for that. Just keep doing damage after the two buffs he does.
This is where he spawns a little guy that runs towards you, you need to run away from him as he will one hit kill you. Running alone won't work here, you need to dodge and charge/teleport at the right time. Watch for the hands of the little guy -- when they start glowing he will rush towards you. First you want to simply active dodge away from him, makes him lose his charge. However, he will do it one more time and that's when my active dodge was on cooldown and I used my charge/teleport -- Flicker (teleports you 10 meters forward, low CD). I believe that's when the little guy will just disappear into thin air, but soon enough there will be another one if I remember correctly. Just do the same thing and you'll be fine, but don't forget to damage the boss while doing it all!
A special thanks to the TSWform user GeZoR
Thanks to him i got this build and phase description