It's from 18 USC 249: Hate crime acts which pertains to

1) Offenses involving actual or perceived race, color, religion, or national origin.-

Whoever, whether or not acting under color of law, willfully causes bodily injury to any person or, through the use of fire, a firearm, a dangerous weapon, or an explosive or incendiary device, attempts to cause bodily injury to any person, because of the actual or perceived race, color, religion, or national origin of any person-

(2) Offenses involving actual or perceived religion, national origin, gender, sexual orientation, gender identity, or disability.-

(A) In general.-Whoever, whether or not acting under color of law, in any circumstance described in subparagraph (B) or paragraph (3), willfully causes bodily injury to any person or, through the use of fire, a firearm, a dangerous weapon, or an explosive or incendiary device, attempts to cause bodily injury to any person, because of the actual or perceived religion, national origin, gender, sexual orientation, gender identity, or disability of any person-

Towards the bottom, there's a section that says

(c) Definitions.-In this section-

(4) the term "gender identity" means actual or perceived gender-related characteristics;

https://uscode.house.gov/view.xhtml?req=(title:18%20section:249%20edition:prelim)

Which obviously isn't very helpful.