The image is in indexed mode - change to RGB Image -> Mode -> RGB
Give it an Alpha channel Layer -> Transparency -> Add Alpha Channel
Color select the black
Invert the selection Select -> Invert
Cut the selection Edit -> Cut
Kill the selection Select -> None
Restore the anti-aliasing Filters -> Blur -> Gaussian Blur with values 1 or smaller ( 0.8 works - enter that by keyboard)
The other way is just recreate the shape with a path, then stroke the path (but I was told off the last time I suggested that
)
Give it an Alpha channel Layer -> Transparency -> Add Alpha Channel
Color select the black
Invert the selection Select -> Invert
Cut the selection Edit -> Cut
Kill the selection Select -> None
Restore the anti-aliasing Filters -> Blur -> Gaussian Blur with values 1 or smaller ( 0.8 works - enter that by keyboard)
The other way is just recreate the shape with a path, then stroke the path (but I was told off the last time I suggested that
)