I know the method to do sign out from all devices on a computer. (Go to Gmail page, in bottom right, click Details, and click Sign out all other web sessions)
How do I do this if I don't have computer and just have my phone?
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This option is only available using a browser as documented in the 'Sign out of all sessions' of this article.
I just tested and this option can be found in a mobile browser. I used Chrome on my Android phone.
Start by going to mail.google.com. Select the menu icon on the left, scroll to the bottom, choose the Desktop link, then follow the same steps you would use from your computer.