Libre Writer recognises 'Garamond' font when loading a document written in it, but does not have it as a selection

When foreign documents use an unknown font you don't have installed, that font's name still gets displayed in LibreOffice Writer, but it automatically picks one of your installed fonts that it thinks is most similar to what you might want to see.

So what you describe is the perfectly normal and intended behavior in the situation that you just don't have any font called Garamond installed.
Note that the substitiution is done by LibreOffice and not by Ubuntu.

As the original Garamond font is not free available, chances are low that you will ever encounter them in any Ubuntu repository.

However, you may find a free clone of the Garamond fonts, EB-Garamont to download here, but note that it is a project which is still in development.

For instructions on how to install font files to your Ubuntu system, please visit the question How do I install fonts? (AskUbuntu.com).

Then you may either be lucky and LibreOffice automatically recognizes that EB-Garamond is similar to Garamond and may replace it, or you have to set it manually. Therefore, you open any LibreOffice application (here: Writer) and select from the menu "Tools" > "Options..." to get the configuration window below, where you choose the section "LibreOffice/Fonts". Enable the checkbox "Apply replacement table" and enter the substitution rule you want in the listboxes above. You will have to write unknown fonts' names manually, but I think it will recognize it. Set the rule with a click on the green tick button and close the window with "OK".

More information on how Font-Fallback in LibreOffice/OpenOffice works, please visit How do unknown fonts get substituted? (AskUbuntu.com) and the article What is Font Fallback (wiki.openoffice.org)