There's many champions like this. I don't have Seer, do you know how many videos I've clicked on to get help with a "X Fewest turns" only to see the entire team revolves around her?
Godseeker is another one for Sand Devil and given how important Ascension is, it really sucks for people without her too. Then there's Armanz in Arena. Ect ect.
All that being said, I think you're being a bit hyperbolic with words like "requires" and "gatekeeping". Gnut does not "Gatekeep" FK Hard. All the content I mentioned is still possible without said champions, they just make it easier. At least he's still summonable, there's a lot of incredible champs like Nergi Archer that we may never get...
Champions like Neldor (a non-void Epic) can make FK Hard 10 possible without insane gear. This might help those without Gnut and next time Neldor is available as a Progressive Chance Target, select him.