First i would like to mention that i tested it on the three armor types.
The Greatsword (in my opinion) was supposed to be a substitute bruiser weapon, since it’s model and usability, and that’s not what is happening actually.
With Light Armor Set the GS (let’s call it that way) was very good if you aim to kill squishy builds, chasing ranged builds is very nice (projectile defense very cool) but the heavy attack and stance changing mechanic is very unexplored in this build.
With the Medium Armor Set the GS is kinda bad in my opinion, since you don’t have the mobility neither de defense in case you get countered (very slow weapon after all) so the normal bruiser (GA and WH) becomes stronger…
With the Heavy Armor Set you can explore more of the heavy attack and stance changing mechanic and do a good amount of skill sets, but the problem is this way is that you can’t give the 200 con passive. This way, if you build 300 STR you get low dmg because of the 84%/81% scaling with STR and DEX, and if you mix the attributes for more damage you lose a good secondary weapon like GA or WH and even the grit from 300 STR so you get countered more easilly, and the grit passive don’t work like 300 STR grit, not even close.
The final point is, in all those builds, the Stance changing that was supposed to exist in the weapon is completely wrecked (in my opinion).
Afterall GS was supposed to be a primary weapon, because of CD reset, mobility and Stance changing, but in my server most people use it as a secondary support weapon using only Relentless Rush and Skyward Slash.
I guess only a little rework would bring this weapon to what it was supposed to be (again in my opinion).