Make an outline with GIMP, like in Photoshop
How do I add a stroke to a layer in GIMP?
Per the GIMP documentation Edit > Stroke Selection:
The Stroke Selection command strokes a selection in the image. There are two ways you can stroke the selection, either by using a paint tool or without using one. This means that the selection border, which is emphasized in the image with a dotted line, can be drawn with a stroke. There are various options which you can use to specify how this stroke should look.
- Create a new Layer
Make a Selection via Rectangle Select Tool
Go to Edit > Stroke Selection and setup your desired Line width:
Note: The stroke color is been taken from the current Foreground color.