If you want weapons that are in any way grounded in reality, Final Fantasy IS NOT THE SERIES FOR YOU.
The latter games perhaps as I don't consider them to really be FF. Though they do have FF elements, the majority of the games have lost a lot of what made them so great to begin with. To me FF is swords and sorcery, with fantastic castles, mystical lands, knights in armor, dragons, long lost civilizations that utilized airships, and of course an epic story that stands the test of time, etc. You get the idea. From FFVII and onward (with the exception of FFIX), I felt the games have been losing touch with their original identity. It maybe trivial in your mind, but to me whom loved the series from 1, 2, 3, 4, 5, & 6; the latter games, again with the exception of 9, doesn't hold a candle to them. At least FF14 MMO has gone back to that long lost identity for the most part.
Nobody in the FFVII canon has a build that would allow them to wield something like the Buster Sword or Masamune with any degree of deftness, and the Masamune is VERY long, very thin, and to have its weight so evenly distributed across such a thin blade makes it a) a nightmare to so much as PICK UP and b) it would bend and/or break in twain as soon as Sephiroth hit ANYTHING with it.
The Buster sword I don't have a real problem with as such large weapons do exist, though granted, not to such a large size. But a blade of that size can still be used. The handle itself just needs to be longer for it to be practical. And in terms of the Masamune blade? Believe it or not, it's based on a real life samurai sword that is over 5 meters in length! In case you don't believe me, watch this video. Just jump ahead to 2:30 as that's where it starts.
https://www.youtube.com/watch?v=T8g1hbYrMDIOr shall we talk about the fact that Wakka uses modified volleyballs as weapons? Even if they have the weight and density of medicine balls, they'd never do any real damage to the things the characters face. Or that Hope uses a BOOMERANG? That's fine for hunting birds, but anything bigger and it'd just kind of mildly annoy it. Thank the team's lucky stars that Sazh in that game is packing heat. And Vanille uses a... fishing rod? Thing? Wtf do we even call that thing?
All of the aforementioned I'm not familiar with as I've never played the latter games of the series. I've only seen short plays of FFVII and FFVIII while watching my bro tend to them.
At least Adult!Rydia uses a whip in FF4. And we already know from dissecting Castlevania in real terms that a whip, properly used, can do some MAJOR damage.
Oh hell yes! Although Rydia's true strength was her spells and summoning technique. The whip was more for style and fineness, but was still awesome.
So really, harping on the Gunblade's nonrealism is kind of tunnel-visioned.
I'm not sure what you're trying to accomplish here. If you wish to defend the gunblade then so be it. I'm only stating my opinion on the weapon. I'm not offending you personally in anyway so don't go overboard. I do my damnedest not to hurt anyone here as I want to come back here. And I want to have good, friendly conversations with others as well. Okay?