Floodgate is 100% not necessary. You can play 0. Ignore the idiot claiming it's a mandatory 3 of. You can replace it with Wall of Disruption, Windstorm, or Half Shut. Actually those are all better than Floodgate in this deck.
You don't need 3 or even 2. I found that they actually hinder you if you have too many. I have 3, but only use 1. I replaced the other 2 with MWs to counter AG decks. I then reached KOG.
My GB KOG deck. I've tried many builds. I tried running 3 Floodgates and I personally found that's not optimal.
The reason I say this is because your opponent may flood the field and then you might draw a Floodgate too late and lose. With Mirror Wall you can stall them to get Narokius out and 🔥 on them.
I stand by what I said, 3 Floodgates is not optimal or necessary.