Gilgamesh and Shard/dimension hopping.

[Naraku_Diabolos]

This occurred to me yesterday while I was driving in my car that during the StB Hildibrand questline, we see that Gilgamesh opens his dimension portal to escape and Hildibrand goes into it. Then we see Hildibrand in The First when doing Heroes’ Gauntlet.

So if Gilgamesh can create portals to other places like The First, it’s perplexing that we couldn’t use him to help us open a portal to the 13th. Better yet, Y’shtola could even go to him if she ever wanted to see Runar again.

[Layte_Aeon]

Even beyond the fact that Hildebrand is optional content... we don't exactly have a line of contact to Gilgamesh, we have no clue if he can choose where to go via portals and we still had to deal with seperated soul shenanigans with Hildebrand, do we really want to risk that again?

[Cleretic]

Gilgamesh is an idiot.

Like, let's put aside the fact that no, they're not making Hildibrand required content for any main story, that's a recipe to get a whole lot of people to quit. Gilgamesh is an idiot. He doesn't know what he's doing with dimensional travel and never has, in any game. Everywhere he's ever traveled to has been by accident; he's canonically never reached FFV's world again, despite that being his main goal, because he's not even good enough at dimensional travel to retrace his steps. Hell, the exact thing the OP puts forward is evidence enough, Hildibrand going to the First was both unintended and a total disaster that took a wild stroke of luck to resolve.

If Y'shtola somehow found Gilgamesh and learned he was capable of opening portals (already a big ask, Gilgamesh isn't exactly known... well, at all, but especially not for that), asking him to open one to the First isn't a recipe for her meeting Runar, it's a recipe for her meeting Vincent Valentine somehow.

[Enkidoh]

Not only is Gilgamesh an idiot, his whole connection to the Dimensional Rift was accidental in the first place - he was thrown into it by X-Death in FFV after all, and from then on seemed to be stuck to it (there was even an in-joke in Dissidia Duodecium where he arrived in Dissidia's dimension unintentionally, challenged whomever the player was playing, and then the Rift opens up again and pulls him back into it, Gil screaming "NOOOOO!!" all the while.).

So although ol' six arms is synomymous with the Rift, he can't control it nor determine where he ends up. It's all very vague.

And to say nothing of him being connected to the Hildibrand quest line which is not directly connected to the main scenario, as Cleretic mentioned above.