I have always liked gestalt characters because of their ability to mix and make unusual characters with a greater depth to their abilities.  I like finding cool synergies, not in making a character with game breaking scores, but one that feels like an unusual and unique concept.

Do others only view gestalt as a high-powered, game breaking rule set?  or is it something that can also lead to interesting characters with more roleplaying possibilities?

and are people still playing games like this?  or have they become too old system-wise?

I've always thought they were a bit overpowered.  I have seen a couple games try it in 5e, but very few. I suspect because the "skill diversity" rationale doesn't apply when you have subclasses that add spells to non spell casters, and backgrounds that add rogue abilities to everyone. It becomes all about the power when the versatility is no longer a factor.

I remember when Gestalt was the Big New Thing and people were jumping all over it because it was just more of what 3.5 was already good at-- more powerful, certainly, but what I think got people's attention was that it allowed more complicated characters-- and then, yeah, it was a super high-powered variant and people started noticing that, with tier zero and triple-nine casters and other shenanigans.

I don't remember Gestalt ever being as big with the Pathfinder crowd-- whether it was because of the direction that PF went with traits and archetypes, or simply the Paizo fandom turning up their noses at anything published by WotC-- but I simply never saw half as many Pathfinder Gestalt games as I saw 3.5 Gestalt games, even after most of the 3.5 playerbase moved on to either 4e or PF1.

It's about the only way I'll play either rulseset anymore, though I'm not terribly interested even then. I have issues with the 3.X/5e multiclassing rules that Gestalt mostly addresses, but I mostly prefer TSR/OSR D&D-- with all of its modern 3pp support-- to anything supported by the Big Two.

I've only run a little bit of 5e since it came out, and... I'm concerned with the way that 5e is balanced, compared to 3.5, the Gestalt would break it in half. The way Extra Attack doesn't stack in standard multiclassing is a mirror image of how Gestalt would allow a Fighter or a Ranger 11 to stack their extra attacks with Paladin's improvide divine smite. Compare any of the Charisma Mafia combos now... to when they're not giving up their higher-level spells for their... well, Charisma Mafia tricks.

I think, yes, in theory it could have more interesting diversity and the like; but generally I found it was overpowered.

To me, the big clue is that you can't really play a gestalt and a non-gestalt in the same party and it be fine balance-wise.

Now that's mentioned, yeah, gestalt does kind of feel like older version multi-classing, but I would prefer that over Gestalt by a country mile; but find problems finding older system games.

Oh, man, saaaaaaame. I would love to see some Old School Essentials Advanced games with, shall we say, a liberal policy toward third-party races and classes. And round-robin selections of what's in or out.

To me, Gestalts served only one actual useful purpose:  if the GM wanted to run premade modules that called for a well rounded party of 5+ Characters but they only have 2-3 Players and didn't want to do the work to reign the challenges in.  Then Gestalts work amazingly well to create teh kind of Big Damn Heroes needed.