Indeed it uses Cocoa/Win32/GTK+, and if you need drawing, there's a small library called gg. It used OpenGL, but in the future will also support DirectX and Metal backends.
GTK is super slow. Do you have any intentions of using raw X11 or Wayland?
From "Simple GUI app" example - "import ui // Native cross platform ui toolkit (uses Cocoa, win32, GTK+)".