I have this sword but it states "upgrade to grand master level to unlock bonuses - Witcher gear 0/6"
What exactly must I do to achieve this? It would seem that I need to collect six Witcher gear items
but there are many types of witcher school gear and I do not know where this particular sword fits in.

You can't upgrade it. It is a 'reward' from Where the Cat and Wolf Play DLC that scales with Geralt's level when found. It only states about upgrading because it's classed as a Witcher sword. Having said that, if it *was* upgradable, it would be with school of the Cat gear, since that's where it came from.

You must collect all the diagrams/craft all 6 parts of the Grandmaster Witcher gear to unlock the bonuses associated with the Grandmaster Witcher gear. Since the Teigr sword is from the school of the Cat its refering to the Grandmaster Feline gear, but you can't find the diagrams/craft the Grandmaster Feline gear until you get to the Blood and Wine DLC.

Hickory is correct as to where I obtained the sword.
Orilon's post tends to differ in content and a bit contradictory.
Not serious. I will simply commence with the final battle.

It is classed as a Witcher sword, but it is NOT part of a Witcher gear set. You cannot upgrade it; it is a named reward sword, and there are no upgrade diagrams for it.