With default theming, GTK+2 looks like a bad ripoff of Windows 9x. (It's quite ugly, but not quite as bad as FLTK) GTK+3 has much better default theming.

GTK+2 has been obsolete for almost a decade now.

Unfortunately there's still a lot of legacy apps out there, and others where the dev unfortunately picked GTK2 despite 3 being available. Not to mention some GTK3 apps not including the proper themes

"(depending on platform)" ... you are naming the one platform Gtk+ properly supports.

I've demonstrated that Gtk+ is not ugly, and you don't get to claim it is if you choose to develop ugly applications with Gtk+.

If you're doing cross-platform GUIs, "it looks good on Linux!" is not a compelling argument.

GTK still has extreme padding and wasted screen real estate around texts, images, lists and other component. Making a slim version and better designed UIs would probably make a lot of difference.

That's due to the GNOME HIG, which I agree is ugly. But GTK+ itself will render whatever padding you ask for.

It existed before Gnome and I’ve not yet seen any GTK interface that doesn’t seem like a slightly better looking Motif UI.

On you typical laptop 14" FHD display, exactly this padding and sizing looks great (at @1x), while at 14" 1600x900 it still looks very good. Sure, on 1376x768 it looks terrible.