Assuming the file is on your Desktop, using Terminal % mv ~/Desktop/foobar.eml ~/Desktop/foobar.mht
If it asks you to set one up, just skip it. Regarding your question, you don't need to setup a Gmail acccount (or any account for that matter) to read a single. It's basically a text format that pretty much all Mail applications can open eml file is a mail file that is based on the MIME RFC 822 specification.
