- Do a fuzzy-select on the background: click in one corner of the backrground then shift-click in the other corners (and inside the chain ring) to add them to selection.
- Select>Grow by two pixels, then Select>Invert
- Add a new layer, filled with transparency
- Bucket fill the selection on that layer with the required color. This shoul dgive you the silhouette of the watch
- Move that layer below the watch layer (drag it down in the Layer list, or Layer>Stack>Lower layer
Btw, not very hard to have a different color for the dial: