Compiling a test file, this assume field names are the same
Perl6 name : gumbo_node_s, C Name : GumboNode

For   has int32 type : c-size=4 / nc-size=4
For   has gumbo_node_s parent : c-size=8 / nc-size=8
For   has uint32 index_within_parent : c-size=8 / nc-size=4 DONT MATCH
For   has int32 parse_flags : c-size=4 / nc-size=4
For   HAS g_node_union v : c-size=96 / nc-size=104 DONT MATCH
Size given by sizeof and nativesizeof : C:128/NC:128
Calculated total sizes : C:120/NC:124