I’ve followed LibO online instructions 5 times. Tools>Options>Internet>Email ?
The string of instructions dead-ends at “Internet” and goes back to the doc. Why?
On Windows, there’s no manual configuration for email client (which might be needed on other OSes). LibreOffice uses a Simple MAPI provider (mail application) that is registered on system. Thus, a Simple MAPI-capable application must be installed and configured as a default mail application in order to work with LibreOffice. I am not sure that there is a “gmail” Simple MAPI application out there; and unless there is, it will be not possible to use “gmail” with LibreOffice.
Having said that: you may use any normal mail client (like Thunderbird) that can be configured to work with a gmail account, and that will work with LibreOffice just fine.