How to add an email address link in a blog

You just set the URL to that email address.

but doesn’t that just send them to log in to gmail? how do you set it up so that it is an actual email that gets sent?

It automatically has them use whatever mail client they have set as the default mail client on their computer. If you are looking for a contact form which allows them to type data into a field and then click submit to have it send you an email, try looking into some plugins.
https://www.ads-software.com/extend/plugins/search.php?q=contact+form