I don't know if it's superior to the 6.5 M/S.
It seems to be about the same OAL,,same caliber of course,,the case is a little bigger around so more capacity I'd guess.

I don't know the down range ballistics but a good guess is that the Gibbs has at least some velocity advantage. You can see I'm not in to cronographs, numbers and such
Accuracy would depend on the rifle, bbl, bore condition, HS, ammo and lots of things that two different rifles can't possible match each other to show which is better.

I think the whole idea of the Gibbs Magnum like so many other propietary cartridges was just to be able to get 'their name' on it. Not so much an interest in making a round that was better, though they likely said it was. But instead just a marketing gimmick.

..and my gun is a rifle length bbl.
Built on a Greek M1903 M/S using the orig bbl.
Can't recall the bbl lengh right now though at least 24,,maybe 26".

It has an Aldis Bros scope in pincer (?sp) mounts atop it.
Found it at a gun show in Ohio (OGCA) a few yrs ago late on a Sunday.
No takers , I must have walked by it all day Sat and SUnday.
$250 and it was mine.

A kind soul later offered me $300 for it to relieve me of my poor reasoned purchase.
...Take your sad story walkin' bud..