You want to flood fill an area of an image in GIMP to be transparent. This apparently is not possible in GIMP. But it's actually quite simple to do.
Note: This might work on other platforms - I assume it does - but I only can test it under Linux so I won't claim it works on other platforms.
This seems to be to be an undocumented feature (at least at the time I originally wrote this, that being 2015-08-08T09:00:23+00:00). Why they would have this undocumented I do not know but I suppose it was an oversight.
- Make sure there is an alpha channel:
Click Layer -> Transparency -> Add alpha channel (if you know the image already has an alpha channel you can skip that step).
- Select the bucket fill option.
- Configure the bucket fill mode to be ‘Colour erase’ (if you have American English it probably will be ‘Color erase’).
- Make sure transparency is set to 100%.
- Execute the flood fill (called bucket fill in GIMP).
By erasing the colour you’re making the area of the image transparent.