[JamOfBoy]

I suggest giving it an entirely different effect entirely. The WIL into phys def was nice but didn't make much sense - it's a MAGIC gunner, so I thought they'd be inclined to have some sort of resistance to magic.

But, to address the point:

Changing it from WIL to GUI is, frankly, a horrible idea.

50 scaled GUI giving you 10% Phys Def, removing the farshot penalty, making your critical hits do 50% more damage, giving you 10 skill slots, and giving you up to 60 additional hit.
As the kids say? Sheesh.

Even if GUI gets changed sometime in the future to not be so packed full of benefits, this still isn't a good idea - it's capable of facilitating the dreaded dodge tanking Dev fought so tirelessly to nerf. At least with WIL, and old Arcane Gun functionality, since you had to balance or choose between having WIL for extra SWA or GUI for tasty armor-ignoring crit damage, it wasn't so prominent of an issue as you didn't typically have both WIL and GUI in high amounts.

With your suggestion, now with no reason to pick WIL, all Magic Gunners - skill points and slots provided - will get extra physical defence for basically free, if not extremely cheap.