I have been trying to use this tutorial to create eyes with Gimp: https://modthesims.info/showthread.php?t=248688 and I encountered a roadblock where I can't figure out how to fade out the pupil since the tutorial uses an outdated version of Gimp. I have looked online and can't figure out how to achieve that step with the current version of Gimp

(I've got Gimp 2.10 - probably only slightly outdated)

The smudge tool looks like a hand pointing an index finger (same tool as next-to-last tool in the tutorial picture).
The settings for it show up below the tools. It seems there's no "length" setting, but you can play with the settings until you find something that looks similar to the tutorial. "Force" seems to roughly decide the length/strength, and "size" decides how big the brush is.

Maybe I should look at 2.10 version of Gimp. Can't hurt to look at it since I only use Gimp for Sims 2. Thanks for the explanation (I hate it when the version of a program in a tutorial is outdated by the time I look at a tutorial. Normally I can figure it out without having to ask people but in this case they either moved things around or got rid of some of the features).