I played the game late, so all of that content was already there. Was nice to do the gauntlets and inner versions in between playthroughs, but I can also imagine it being a really great addition to come back to later on.
I've completed 3 endings and 3 gauntlets... I've peeked online at the list for the final gauntlet and I'm not sure I'll ever attempt it, my god. The ones I've done were harder than anything else I've ever played and I'm not sure I could even do them again (have only played a couple of other games in this genre)
On the other hand, one day I'll be curious to see the fourth ending, so I'll play it again, then I'll probably just have a wee look at the gauntlet, then I'll see if I can get a bit further, etc.... Shit.