Good post from HH above. Discussion of gender is very confused, since it used to refer to cultural differences, particularly to do with masculinity and feminity, but is now often used to refer to biological sex identity, i.e. maleness and femaleness. Obviously, the first one has a wide spectrum, people can be more or less masculine and feminine. There are butch women and feminine men.
Biological sex identity certainly has 'inbetween' cases, that of intersex people; I don't know if these in fact exist on a spectrum.
I find this writer (in the link) confusing, as she keeps talking about masculine and feminine (the old sense of gender), but seems to also revert to talking about sex identity (the new sense).