The GUI designer is called Qt designer - it's called designer in Ubuntu/Debian. Unfortunately Debian has really made a mess of their packaging of Qt/PyQt. They'

The GUI designer is called Qt designer - it's called designer in Ubuntu/Debian. Unfortunately Debian has really made a mess of their packaging of Qt/PyQt. They've split up everything into tiny packages and their naming scheme seems crazy - I can never work out which package to install to get what I want. In this case it's qttools5-dev-tools.