I don't know why the theory is widespread that it has to be red.
See a simple extreme example:
remove the black circle from the image

when you use red

when you use a color that doesn't exist in the image:

One condition must be met:
the color of the painted area must match (be identical) the one selected in G'MIC!You can simply specify it using "Pick screen Color" and you will get the desired effect.
result

And work on a copy of the image because Inpaint G'MIC is destructive according to some (they don't use Edit ➤ Undo or Ctrl+z shortcut).
