There& #39;s no such thing as "gender identity theory". There is such a thing as gender (masculinity, femininity, whatever) it& #39;s a social construct but it& #39;s real. Growing up in a gendered society, most of us develop a gender identity early on: sense of self as boy/girl & woman/man...
it would be rare not to develop an internal sense of gender, intrinsic to self, because we are taught to be gendered, taught to fit in -so it sinks in. Most ppl have a sense of self as a man or woman, as masculine or feminine, as a husband or wife etc - gendered roles in society.