Yes. The brush editor needs to see colour channels in the brush tip image before it will let you use them for Brush Mode: Gradient Map.
So, you can convert a greyscale colour space brush tip image to RGB/A in krita and then Export it as a .png image for use as a brush tip.
I think I should have used a desaturated yellow colour instead of grey then it would be more clear what they were when looking at them.
I’m sure you’ll sort it all out fairly quickly so please show what you’ve done when you do it 