How to keep the black outline only in a image

[denzjos]

@Meetdilip, its not the question you asked for but you can finetune the result if needed :
- remove colors : Colours / Desaturate / Mono Mixer (use all sliders to get a satisfied result)
- smooth the drawwing with : G'Mic-Qt / Repair / Smooth [Mean-Curvature] (Time Step 30 / Iterations 13)
- duplicate the layer 5 times with layer mode 'Multiply'
- make a new layer 'New from Visible' (layer name 'Visible')
- G'Mic-Qt / Repair / Smooth [Mean-Curvature] (Time Step 30 / Iterations 13)
- duplicate the result to a new layer (layer name 'Visible copy') and set the layer mode 'Addition'
- Merge the layers 'Visible' and 'Visible copy'
- result :