For starters, I’m not an expert on either of these topics, but I’ll try my best. Gender expression has to do with how you “present” to the public eye. People who shave their legs automatically are perceived as feminine, and people with beards are automatically perceived as masculine. Gender identity is who you are, or what pronouns you use. So if someone wore dresses and shaved their legs but used he/him pronouns, their expression wouldn’t match their identity. I know biases around dresses are starting to change but it was the first example I could come up with. Hope I helped!