That’s what I just came to say. I see I’ve got both GTK3 and GTK4 installed:
$ pamac search gtk3
[...]
gtk3 [Installed] 1:3.24.38-1 extra
GObject-based multi-platform GUI toolkit
and
$ pamac search gtk4
[...]
gtk4 [Installed] 1:4.10.4-1 extra
GObject-based multi-platform GUI toolkit
So perhaps install the one you don’t have?
pamac install gtk3
Or
pamac install gtk4
