A mass noun refers to a noun that cannot be counted.

This plan is impossible.

Unexpectedly, his face came to mind.

His nature is very kind.

Detailed information is needed to understand the nature of that project.

I was moved by his sincerity.

His nature is very kind.

Gender identity refers to the gender that each individual recognizes themselves as.

He is always joking around during class.

His irregular lifestyle is having a negative impact on his health.