How to fill entire image from gradient outline

[denzjos]

I little time consuming but I tried this :
- made a layer copy from the original
- put a white layer under the copied layer
- made a new layer 'New from Visible'
- select the inside of the drawing with the 'Fuzzy Select Tool'
- 'Select / Grow ' (20 pixels)
- 'Select / To Path' 
- 'Select / None'
- make a new copy from the original and work on this layer (an set the other layers not visible) 
- 'Filters / Blur / Median Blur (Neighborhood Circle, Radius 30, Percentile 50, Alpha percentile 100)
- 'Select / From Path'
- delete the inside of the drawing
- 'Select / None'
Then until the square is filled, repeat :
- 'Filters / Blur / Median Blur (Neighborhood Circle, Radius 30, Percentile 50, Alpha percentile 100)
- 'Select / From Path'
- delete the inside of the drawing
- 'Select / None'
Merge the result with the original layer
Result :