I need to know how to trace patterns along paths, like in this image:
I've tried to stroke the selection with pattern, but that obviously doesn't care about any angles and only works like a mask.
GIMP isn't really the right kind of software to do that. You'd be better with vector image editing software such as Inkscape, which is also free and open source like GIMP. Or if you have money to burn, there's always Adobe Illustrator.
When you see graphics like that, these are nearly always done in vector image editing software, not using raster image editors such as GIMP/Photoshop.
There's a tutorial here to create a rope pattern along a path using Inkscape. Inskcape has a specific Pattern Along a Path effect for those type of effects.
Here's an example I made using that tutorial in Inkscape, although my attempt is a bit crap. I'm sure you could do better with more care.
You can, if you replace your pattern by a brush (and you can always use the
clipboard brush (first in the list), after copying the relevant image to the clipboard). Then:
Edit>Stroke path, select
Stroke with paint tool
Obviously, if you want a continuous pattern, and if it needs to be distorted in the curves, this is a whole other problem.