I had a good think about how to do this The Unix Way™ and my best sugestion would be to have a script that monitors a folder for screenshots and launches a program (gimp) when it sees something. I wrote one, tried it out, and it works really well!

For this, I used inotifywait which you can get by installing inotify-tools (at least, that’s what it’s called in Arch):

#!/bin/bash

inotifywait -m "${HOME}/Pictures/Screenshots/" -e create -e moved_to |
    while read -r directory action file; do
        if [[ "${file}" =~ ^Screenshot.* ]]; then
            gimp "${file}"
        fi
    done

All this does is use inotify to trigger an action whenever a file is created in the folder (in this case, the Screenshots folder in ${HOME}/Pictures). For our purposes, it just looks for files named Screenshot and if one appears, we launch gimp.

The result is that if you run this thing and start screenshotting with GNOME’s built-in tool, each action will trigger gimp. You can then further expand this to perform some sort of custom action (either with a gimp macro or with some Python script, whatever) so that whenever you’re running this script, it’ll take that screenshot and do something to it whenever it’s created.

Does this help?

Couldnt you just put a custom script onto the print button to take the screenshot and send it to a light editing program?
I have my normal screenshot button and another one which afterwards send the selected region to img2txt and puts the detected text into the clipboard.