You know, I totally didn't think about that before I posted this. It might actually be possible...trying to get a friend with NIN and SAM to take a look at this.
I've used a GK on my NIN before. Even went out of my way to get a Sukesada waaaay back when.
The first time for the "lolz" and to cap all my combat skills (because I could) and quite recently for WS weakness triggering (although I haven't actually triggered it through GK weapon skills yet - other weapons have taken precedence).
Use it on NIN for red Abyssea weaknesses. NIN can use jinpu and koki
As a dual combo, Ninja and Warrior get Access to all the Atma (red) Sourcing weapon skills. It makes life a lot easier, especially if you're trying for a TIII Abyssite change, since Red !! will actually make a TII->TIII change 100%. (All key items 100% apparently)
Use it on NIN for red Abyssea weaknesses. NIN can use jinpu and koki
As a dual combo, Ninja and Warrior get Access to all the Atma (red) Sourcing weapon skills. It makes life a lot easier, especially if you're trying for a TIII Abyssite change, since Red !! will actually make a TII->TIII change 100%. (All key items 100% apparently)
I use it on Key item NMs and get access to 10/13 weaponskills for red !!. Yeah it is 100% key item.
I use sword, dagger, GKT in additon to katana for red weakness. Makes it so the Warrior I play with has to bring two less weapons. I should probably go skill up my club, too.
Thank god they equalized the skill ups more after 75 cap. I can't imagine falling behind like 4 more skill levels every single level all the way up to 99.
As for new weaponskills, I can get Aeolian Edge on dagger with merits, so I'm guessing Tachi: Ageha will be available to us. Not that it'll do us any good.