Gnomish whetstone is a crafting component in The Witcher 3: Wild Hunt that is needed to craft master's weapon repair kits. It is not needed for anything else nor can it be dismantled.
It can be purchased from the following merchants:
- All blacksmiths in Novigrad, Oxenfurt, Skellige and Toussaint, except the one at Ardaiso Quarry
- Merchant at the Isolated Hut on Hindarsfjall