I don't think trans is expression I am against that ideology. However professionals and the community will tell you otherwise. You can be diagnosed with gender dysphoria for having an expression, you don't need any actual symptom requirements.

Also when you say gender identity which definition of gender do you refer to?

gen·der /ˈjendər/ Learn to pronounce noun 1. either of the two sexes (male and female) 1, especially when considered with reference to social and cultural differences rather than biological ones. 2 The term is also used more broadly to denote a range of identities that do not correspond to established ideas of male and female. 3

When you say gender identity what is that? Which definition of gender are you using because the definition of gender mentions 3 distinct different things. If you ask me trans is about sex, but clearly to most of the community it is about culture which yes is expression or identity which they don't even explain what that is so I guess it can be anything at all, which means expression and not something physically real they are identifying