Contour light

[Krikor]

The image already has a transparent background.
01- Layer - Transparency - Alpha to Selection;
This will create a selection around the image.

02- Select - Border;
Choose the width of the border, something like 3 px could work.

03- Create a new layer above the figure, and make it active.

Now you can simply:
A- Select a color in the active foreground color and drag to fill this selection.

Or

B- b1- Edit - Stroke Selection... and then Select - None.
b2- In the Paths tab, select the created path and right-click and select - fill Path - Solid color.

Obs: An advantage of using paths is that you can make small adjustments if the edges are too offset.

If necessary you can change the color via Color - Colorize.

   
By changing the layer modes it is possible to obtain a better result of the color used on the figure.