You're misunderstanding the level at which html5lib operates: it merely parses to a tree (using a "tree builder" to provide a common API to the parser to build

You're misunderstanding the level at which html5lib operates: it merely parses to a tree (using a "tree builder" to provide a common API to the parser to build the tree, which can be a DOM tree, an ElementTree, an lxml tree, whatever) and provides a generic "tree walker" API that walks over one of those tree formats and provides a common stream of events (start tag, end tag, text, comment, etc.) which can then be used, e.g., in the serialiser. This can therefore be used with Gumbo by passing the lxml tree builder into its html5lib.parse like method.