wpgraft never reads Gutenberg’s data. There is no exporter and no importer anywhere in this. It reads the page as your browser rendered it: the real elements and the real computed styles on each one, at every width you ask for. It then builds that as a Shopify section made of theme blocks, with every style in one of the blocks’ own settings. Nothing that arrives depends on Gutenberg still being there.
Moving a WordPress shop to Shopify means rebuilding the marketing pages around it, and core blocks have no export a Shopify theme can read. wpgraft copies the rendered WordPress page and writes it into your theme as a Shopify section of blocks. Your WordPress site keeps running while the store is built.