There are a number of options for adding a GUI to Babylon.js. A popular one used in the Playground is dat.GUI, an external interface but is integrated within the Playground.
In the extensions you will find reference to Canvas2D. This extension made use of the GPU and was growing in popularity however for personal reasons the author can no longer support nor develop this extension.
The recommended GUI is now the extension BABYLON.GUI and has buttons, labels that adhere to a mesh and a color picker among other features.
Playground Example Slider dat.GUI -