I could not find any options in the web interface of Gmail.
All I could find was a Firefox plugin to use S/MIME with Gmail, but it seems to be outdated.
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There is Penango - https://www.penango.com/products - which seems to have a free edition for GMail. I looked into it a while back, since as you say, there isn't anything all that up-to-date, but haven't tried it (ended up going with S/MIME in Outlook checking my GMail account).
Another option is Fossa Guard - S/MIME extension for Google Chrome browser. S/MIME certificates are free. And it is interoperable with many other mail clients.