GTK is nice but too thick. There is a repo containing CSS for GTK3 to be more compact.
I could not find the same for GTK4.
Gradience from Flathub allows to load custom CSS. The GTK3 one did not change anything.
I use Fedora Kinoite (KDE) and all GTK apps are Flatpaks.
Gradience mainly changes the looks of libadwaita applications and it only work on gtk3 and gtk4 apps if you use the adw-gtk3 theme.
There is an important difference between gtk4 and libadwaita. While libadwaita apps use gtk4 in the background, they cannot be themed through normal gtk theming.
I don’t know of any way to change the compactness of gtk4 or libadwaita, sorry.
It also seems to poke a hole in all Flatpak apps, giving them access to GTK3 themes.
Thanks, maybe it is not even possible? Should ask GNOME devs…