Convert a photograph partly to Grey

I want to convert a photograph to grey-values (8-bit) except for 1 part, how to do this?

grtz

Make clone the photo (Edit > Clone). Edit the clone with the Photo Tool and reduce the saturation to 0 (grayscale).

Select the edited clone with the Transparency Tool (the wine glass icon). Click on the photo and drag downward (or sideways) to apply a Linear transparency.