Comment # 13 on bug 1191402 from Mrcuve0

(In reply to Jebin S from comment #10)
> The issue stopped once I disabled export GTK_USE_PORTAL=1 in the profile
> (which is a custom change I had made!). Now, they work fine. I'd still like
> to know if there's a way to get kde file selection in gtk apps in KDE! I
> think this bug can be marked as NOT a bug if no else is facing this issue
> right now!

Can confirm, I had "export GTK_USE_PORTAL=1" in my .bashrc. I commented it,
logout and the problem is solved (w/o having to reinstall
"xdg-desktop-portal-gnome" like I did before).

At least on firefox, you can get KDE's file picker by setting
"widget.use-xdg-desktop-portal" to "true" in about:config. This solution is
temporary and limited to only Firefox however