convert a layer to greyscale

[Ofnuts]

Also, the weird recipe:

• set the bucket-fill tool to "Saturation" mode
  
• bucket-fill the layer with white
  

Why it works:

• Alongside the usual RGB model, another possible color representation model is HSV (Hue, Saturation, Value), saturation  being how "colored" things are.
  
• By setting the tool to "Saturation" mode, only the saturation of the painting color is used, in other words in each pixel, RGB is convert to HSV, S is set to 0, and HSV is converted back to RGB.
  
• And removing the saturation is one way to make everything gray.