I cannot agree more, this sounds extremely daunting and exhausting just for a screenshot.

BTW I'm using Ksnip (sudo apt install ksnip), crossplatform over there -> https://github.com/ksnip/ksnip
It has all the tools needed like aut-numbering, arrows, double arrows, rectangle, circle, annotation, highlight, blur, pixelize, and so, and so, also like draw a rectangle to screenshot that part, it recall always your last setting, and so much more like opening a png or jpg independently to put arrows or whatever you want.

a screenshot of that screenshot app

I use a free version of PhotoScape X for windows.http://x.photoscape.org/

PixLab, it's not even "just for a screenshot"
but for highlighting text with a yellow background, aka annotation.
Into the bargain, the subject is "Using Gimp" but not an external tool such as GreenShot.

However, according to proof by contradiction,
AlSchemist will supply the usage with the external tool GreenShot quoted by IMZvonko for comparison.
Let us suppose that "to simplify" the screen capture or any image is already loaded inside Gimp.

1. In Gimp, press Ctrl+C to copy the picture in the clipboard
2. Click the GreenShot's green icon in the taskbar
3. Click the contextual menu "Open image from clipboard"
4. In Greenshot, select the Highlight (H) tool in the left margin
5. Draw a rectangle around the wished text: it is not possible to make rounded corners.
6. Press Shift+Ctrl+C to export the highlighted image in the clipboard
7. Come back into Gimp and press Ctrl+V to paste the highlighted image from the clipboard
8. Click outside the image to merge the "Floating Selection (Pasted Layer)"

Alternatively GreenShot would be able to run Gimp but the loading of Gimp would be too slow.

With the Script-Fu HighlightText.scm script in Gimp:

What was missing in the AlSchemist's very long post is the [spoiler]...[/spoiler] tag to hide the waffles.

HighlightText.scm is for beginners who wish to learn not Python but gimpy ana--thema--tized Scheme of course without changing the settings of the

Ah, that was a script.... Btw, no need to restart Gimp after installing a script-fu, just do "Filters > Script-fu > Refresh scripts"

Let us suppose that "to simplify" the screen capture or any image is already loaded inside Gimp

Of course not. The deal in that case is that you can do everything without using Gimp.

This said to highlight something in yellow in Gimp, set your FG to yellow, your brush to Multiply mode, adjust the size of the brush, then:

1. Click on the start of the highlighted text
2. Depress shift, and move to the end of the text (you can in addition depress Control to make sure the highlighting is horizontal) then click again.

Of course, you can save your brush settings as a preset:



So highlighting is just restoring the preset and a couple of clicks.

AlSchemist
Sorry, I felt I was obliged to do it

More seriously (but not so ) let's get back on track.

So it's even worse than I thought....
Not only you are cluttering the top menu with a script that needs 1 more click than doing it WITHOUT script, but also that person will not learn GIMP properly nor use the proper tool to do it.



So to highlight something on a screenshot in GIMP the OP will always be dependent of you?



Oh my... you're not simplifying, at all!
BTW your Click outside the image to merge the "Floating Selection (Pasted Layer) is wrong! clicking outside the image will NOT merge the pasted/floating selection (and happily it does not do this!)

Also I just learn that on Windows OS when you hit the "PrintScreen / PrtScr" key, Windows copy it to the clipboard (yeah.. I needed to do some searches -> I'm using only Linux)


In the end why all those copy/paste and back and forth between greenshot and gimp?
Why not Highlight directly in GIMP as the OP asked?
Hit that "PrintScreen / PrtScr" key = clipboard, then in GIMP > File/Create/From clipboard, then >> New layer in multiply mode >> make your selection >> drag n drop the color from FG or BG >> DONE! and this method can protect from mistakes as well...
Or use the brush as suggested by @Ofnuts, big advantage of Ofnuts brush, you got a preset that you just call once and highlight as many time as you need just with a brush, also next time when you re-open GIMP it's there and accessible in just 1 click.


OK, it's a GIMP native tool

OK, it's a GIMP native tool

No script needed for that >> Just drag n drop the FG or the BG color on the image...
Oh you DO NOT EVEN NEED to drop it in the selection as it will fill ONLY the selection and if you use the multiple selection mode (addition/add) it will fill all at once,


OK that's nice, as an exercise it can be good to learn tiny script,
but as a utility script, no, it's not even re-inventing the wheel, it's making things more complex as they should be.

It's amazing how around twenty tiny lines of Scheme could generate as secondary effects.

PixLab, AlSchemist particularly appreciated the pythonesque quote of the movie!
Into the bargain, you mentioned about ksnip the key feature of auto-numbering enough interesting also in GreenShot.



Ten years ago, Silas Silva created mark-number-circles.scm in github under GNU license.
In the 2021 version, some original "TODO" have been fixed by AlSchemist.


Usage:

1. After unzipping, copy mark-number-circles.scm 5 KB in C:\Users\YourUserName\AppData\Roaming\GIMP\2.10\scripts\
2. Restart Gimp
3. Draw two or more points with the Paths Tool
4. Click the Rectangle Select Tool
5. Gimp menu "Filters" > "Render" > "Mark Number Circles"

Each point of the path will be replaced with a incremented number inside a circle.

Hereafter enclosed is the process of migration in Gimp 2.10.28 because in the decade, some API were deprecated.
The original gimp-path-get-points is "deprecated: Use 'gimp-vectors-stroke-get-points' instead." says PDB aka Gimp Procedure DataBase.
The old API generated a longer vector translated into a list, hence:

About the drop function, Silas indicates:

It is the very first time that the abbreviation SRFI 1 standing for Service Request Feature for Implementation is ever written in GimpChat.
Hope that AlSchemist will be not burnt alive on the stake for this taboo.
That is why the function drop of SRFI 1 has been dropped in the 2021 version of mark-number-circles.scm.

After all, for numbers in circle, there is SRFI 14 Unicode Character-Set.

To be continued.