As you don't say exactly what you've done and how far you got, I may be telling you things you know already.
First, "Mail Merge" is traditionally a process where you stuff data from a data source (database, spreadsheet, whatever) into fields in a document, creating a number of identical-sounding letters to a list of recipients. We're talking physical letters here, as in documents that get printed out and stuffed into envelopes. Hence, if you do a Google search for "mail merge", the various tips and tutorials that turn up may not mention bulk e-mail functionality at all.
Second, your mail client is not Gmail. Your mail service
may be Gmail, but in order to send bulk e-mails from an application, a mail client of some sort has to be involved. A web-based interface will not work.
Usually, bulk e-mails are sent using a locally installed mail client (Thunderbird, Outlook, whatever), not a Word Processor. In order to send bulk e-mails directly from LibreOffice, LibreOffice Writer would have to interact with the default mail client on your system, or act as an e-mail client itself. I was somewhat surprised to learn that Writer seems capable of doing the latter
You asked if anyone has successfully done mail merge with e-mails in LibreOffice. I myself have not, but as there are specific settings in Writer for doing just that, it's probably possible.
If you were a bit more specific about what you've tried, you'd be more likely to get a response from someone who's actually done this (but who may not feel up to writing a 5-page tutorial in response to a non-specific inquiry).