Let me start off by saying Gauntlet: Slayer Edition by Arrowhead Game Studios, if you're not familiar (because I know you're familiar with Hades) is one of my favourite games, I wholeheartedly recommend it, it is a superb hack'n'slash. Sure stylistically Hades and well let's face it all of Supergiant Games games are stunningly slick in presentation the term eye-candy damn near needs to be redefined. There are honestly many differences and we could talk about these slight difference for days, but I'm not here to nitpick. But one thing I think we might come to agree upon is that the bones of a damn near perfect hack'n'slash game they both share. In this respect, they're twins, with really only one fundamental difference. Let me be quick to it: it's their progression. Hades with a more recent innovation being rogue-like becoming stronger as you play more as opposed to Gauntlet's skill-based progression i.e. becoming more skillful as you play more.
It can also be said that both games sadly also share fundamental miss-steps in their progression where Hades is not immediately visible that you have "leveled-up." It is only through playing the game that you get a feel for it, a slight miss-step sure. Where as Gauntlet's progression is not visible in the developers intention to help you progress at a gradual vertical pace and what I mean by that is they drop you in the deep-end. Let me explain, sure there are difficulty settings but the last boss is head and shoulders above all else in difficulty only in endless mode does it offer up a challenge worthy, but that's a story for another time.
So how do these two titans differ? I think I have stated my case it's how they go about progression: Hades is more leveling-up in a rogue-like way and with Gauntlet it's in a skill-based way. Not so subtle differences but one maybe more digestible than the other and I think that reflects in both games popularity.