All sorts of ways to make a frame. There are some essentials which apply to most images.
Work on layers. If your frame is on a layer with solid background make a new transparent layer. Create a selection from the solid frame - any number of ways to do this using selection tools.
A couple of screenshots that might help.
https://i.imgur.com/zSHj65g.jpg
1.
Selection -> Distort with small values. Depends on your image size to a certain extent. Does what it says.
2. Fill the selection with some pattern, or paint in it. This is what Gimp is best at,
User Decides Plenty scripts/plugins will give a result, but it is not your concept. This one click-n-drag a pattern in.
https://i.imgur.com/tToYfQJ.jpg
3. That is a bit too light for my taste so darkened it using 'Colors -> Levels' But could equally coloured it in using
Colors -> Colorize
4. That might be sufficient or here a small blur applied
Filters -> Blur -> Gaussian Blur Finally kill the selection
Selection -> None
Do not limit yourself to this procedure, best way in Gimp is experiment.