According to Wikipedia[1] GtkFileChooser is from 2.4 in 2004. So that was designed more than 16 years ago. Not sure when Windows got the feature, but previews could have been thinkable then and machines powerful enough to handle it with some usable threshold.

[1] https://en.m.wikipedia.org/wiki/GTK

They're actually well on their way to switch to GTK3. I guess it takes a while.

https://www.gimp.org/news/2020/11/06/gimp-2-99-2-released/

GIMP is also one of the most atrocious UX I've come across