If I understand things correctly, gumbo in html5-parser was forked from https://github.com/Sigil-Ebook/sigil-gumbo/ at 0830e1145fe08758d6ef24f77dfcbeac4633676f, which in turn was forked from https://github.com/vmg/gumbo-parser/tree/v1.0.0 which was forked from google/gumbo-parser, pre google/gumbo-parser 0.9.4.

There's quite a few changes on the original upstream since then. gumbo_malloc -> gumbo_parser_allocate, free_node -> destroy_node, passing around the parser as an argument, &c.