I'm logged in my Gmail account all the time with my cell phone. Tonight I lost my cell phone and would like to log off on all devices. Can I do this from my home computer after I log in to my Gmail account?
Yes you can, extracted from the following Gmail help article.
Forgot to sign out?
If you've forgotten to sign out of Gmail on another computer, you can sign out of your other sessions by going to the bottom right corner of Gmail, clicking Details, and then Sign out all other sessions.
You could also go to this Devices & activity page and remove the access for any device.
If you use the Gmail app, the solution made by pnuts would't work. The only option then is to check if you have remote access via Android Device Manager (https://www.google.de/android/devicemanager) enabled, so you can delete everything that is on your phone.