Select By Color Tool

• Add transparency layer
• Select by “Hue”
• Threshold about 15

One internet site says this:

Steps to remove the Green:

1. Always make a copy of the original photo, just in case you really mess up you still have the original.
2. Open your photo in Gimp.
3. Go to Layers and Add an Alpha Channel.
4. In your Tools choose the Color Pick Tool.
5. Click on the Green background. …
6. Go to “Edit” and choose “Clear”.

http://www.diaryofatechiechick.com/2013/04/removing-green-screen-as-easy-as-1-2-3.html

I think I did something similar but changed selection to “hue”