The alternative is to use... some pen and a paper? > . If you need to mock something up, you can try sketching on a piece of paper or using our mockup resource

The alternative is to use... some pen and a paper? > . If you need to mock something up, you can try sketching on a piece of paper or using our mockup resources. Linux GUI is honestly in a bizarre state. And the Gnome team seems to keeping on recommending against the tools they developed themselves (Vala...). Seems like QT is the best alternative...