It’s because the blank lines after each paragraph is an empty <p></p> tag. Gmail often strips those empty tags out. If you do a Shift+Return after each paragraph, it will put a <br> tag in there, which Gmail will leave alone. 

You could also put a space on a blank line (which would look like <p>&nbsp;</p> ) and that would work, but for some reason, Gmail may change the text to purple.