Hi Mike,
As far as I know, you can only send a file from the Finder to the Mail app., where you would address and send it.
In the Mail app, you would choose to send in Gmail, Yahoo, or whatever e-mail account(s) you have set up.
The Mail app can incorporate Mail from all and any E-mail accounts that you may have, if you set that up.
(Eg. my own Mail app. has Gmail, Yahoo, and my NBN provider all linked, so mail arrives, and is sent, all from the one app.)
If you click on the little black Apple icon at top left of the top menu bar, and open System Preferences, then click to open Internet Accounts, that is where Gmail would be listed, as Google.
"Internet Accounts" is where there is help set up any e-mail accounts for you, that you wish to access in the Mail app.
Other than that, you would access your Gmail directly from Google / Gmail on the Internet with the Safari app.
https://mail.google.com/
Gmail online, and the Mail app on your computer can both be used to read and send eMail.)
All the best