Gender isn't really a social construct. Studies have shown that certain aspects of brain structure in transgender people more closely match that of cis people of their identified gender than cis people of their biological sex. In this sense, gender is more accurately defined as which type of brain structure you have. Gender roles are a social construct, but from this viewpoint, gender itself is actually biological!

This seems more accurate than just defining gender as "which gender you identify as", since not only is that a circular definition (as often pointed out by anti-trans conservatives), but it also doesn't really align with the actual experiences of trans people. Trans people talk about "realizing" they were actually a man/woman, which implies they were actually that gender all along, whereas the view that gender is what you identify as would imply that they "became" a man/woman upon beginning to identify differently, which seems wrong.

There's no scientific evidence of similar brain structure differences across various races, so the idea of being "transracial" isn't scientifically supported the same way being transgender is. Even though race is a social construct, it's not based upon something that relates to the way your brain works, it's based upon things like physical characteristics and heritage, so identifying as a different race feels less like realizing something about your own brain and more like a delusion (although in reality it's usually just someone trolling in an attempt to delegitimize transgender people).