11-23-2019, 11:53 AM
A bit short on how you obtained the selection 
but it might go like this: Check on the screenshots as you go along.
https://i.imgur.com/2rMNTWs.jpg
Making the selection: For an image with a transparent background you might use Layer -> Transparency -> Alpha to Selection
Stroking that selection either on the layer or on a new layer under fills in the small transparent areas in the foreground.
https://i.imgur.com/TRju1kd.jpg
You need to 'lose' those small areas of selection. One of the Gimp Selection tools is 'quickmask' Either Select -> QuickMask or shift-Q or the little square icon bottom left in the Gimp window. Three ways of getting it. That is how important it is. With the quickmask on transparency shows as red. Paint in the canvas with Black to make a selection. Paint in the canvas with White to remove a selection. The X key swaps FG/BG colours Assigned for the purpose.
With white brush paint out those litttle selected areas. Toggle the quickmask off. Now you can stroke the selection for just the outline.
but it might go like this: Check on the screenshots as you go along.https://i.imgur.com/2rMNTWs.jpg
Making the selection: For an image with a transparent background you might use Layer -> Transparency -> Alpha to Selection
Stroking that selection either on the layer or on a new layer under fills in the small transparent areas in the foreground.
https://i.imgur.com/TRju1kd.jpg
You need to 'lose' those small areas of selection. One of the Gimp Selection tools is 'quickmask' Either Select -> QuickMask or shift-Q or the little square icon bottom left in the Gimp window. Three ways of getting it. That is how important it is. With the quickmask on transparency shows as red. Paint in the canvas with Black to make a selection. Paint in the canvas with White to remove a selection. The X key swaps FG/BG colours Assigned for the purpose.
With white brush paint out those litttle selected areas. Toggle the quickmask off. Now you can stroke the selection for just the outline.