But what happens if someone sends you an email from a non-gmail account? Can you react then?

If so, does it just reply to this email with an emoji in the body? Cause then you’re basically just replying in the exact way as before, google just added a quick-reply button with a predefined body.

I’m personally not a fan of nonstandard functionality for something as ubiquitous as email. Email should be exactly the same regardless of the client that’s used.