I am on level 26 and have the Teigr steel sword equipped.
In green letters it states "Witcher gear 0/6 - Items must be upgraded
to grandmaster level to unlock bonuses"

Now I have no idea whether doing this it is worth the effort for whatever bonuses
are provided and where do I start? It does not state whether this sword is from
the Wolf, Cat, Griffin or Bear school.

I thought as much, but don't understand why on pointing on this sword the above message is given when it does not apply to this sword.

Somewhat off the subject: I have many crafting diagrams like Enhanced Griffin silver sword, master crafted items, yet
I have never crafted a single item as they all are shown to be inferior to what I have. For this reason I no longer go looking for witcher school gear.