These docs are incomplete, and likely to be wrong. Gum doesn't even have an implementation yet. Things are constantly evolving and changing.
Welcome to Gum's documentation! Gum is a text-based programming language that compiles to Scratch 3; more specifically, it was made for creating SN-Edit projects. Gum is designed to be more advanced than Scratch, and so it may have a higher learning curve, but every program made in Gum is still completely reproducible in Scratch.