Select the white backgroung with the 'Fuzzy Select Tool'
Inverse the selection : 'Select / Invert'
Then 'Edit / Paste as / New Layer'
Another option is to copy the layer to a new layer
Then : Colours / Colour to Alpha
Inverse the selection : 'Select / Invert'
Then 'Edit / Paste as / New Layer'
Another option is to copy the layer to a new layer
Then : Colours / Colour to Alpha