convert a layer to greyscale

[Ofnuts]

Colors>Desaturate with three standard choices of color conversion, or, if you feel more adventurous, Color>Components>Channel mixer (and tick the "Monochrome" check box). In addition to working in a single layer, they will also take the selection in account.

<pedantic>Either choice gives you a gray layer (3 bytes per pixel, with R=G=B) but not a grayscale one (one byte/pixel)</pedantic>