MHL-Ruler is a Script-Fu plug-in for GIMP 3.x that creates a proportionally divided ruler from the current selection.
It generates a layer group containing evenly divided sections, which can be used for composition guides, proportions, measurements, etc.
Copy mhl-ruler.scm to your GIMP scripts folder and restart GIMP.
Tip
You can find the script folders in GIMP preferences:
Edit -> Preferences -> Folders -> Scripts
Open an image and create a selection, then run:
Filters -> MHL -> Ruler
Adjust the settings as needed and confirm.
The previous version for GIMP 2.x is available in the
gimp2-legacy/ directory.