Cant Close Windows, app already quit I am running a 2019 macbook pro with GIMP 2.10 installed

For some reason, gimp's windows will stay open, even after the app is quit

how can I close these windows?
I face similar problem with octave-GUI. 

I run 

killall -9 octave 
killall -9 octave-gui


twice to get rid of that.

So you can run 

killall -9 GIMP


Make sure you're using case sensitive name of the app.