Desaturate certain portion of color using Gimp

Method 1
1. Open image.
2. Duplicate layer.
3. Select Duplicated layer. Click Color-Desaturate-Average-OK.
4. Right click on BW layer, add Alpha Channel.
5. Use various tools to select portion which you want to keep the color.
6. Click Eraser tool to erase the selected portion.
7. Flatten image.

Method 2 (Recommanded)
1. Open image.
2. Duplicate layer.
3. Select duplicated layer. Click Color-Desaturate-Average-OK.
4. Right click on BW layer, add Alpha Channel.
5. Right click on BW layer, add Layer Mask.
6. Select WHITE layer of layer mask.
7. Make sure Black foreground color is selected.
    Select Paint brush begin to paint the area you want to be in color.
8. Flatten Image.

NB: If you overdo the boundary and can't undo, switch to white paint and paint opacity back over your mistake to repair the mask.