True, but the GPL wasn't widely used until Linux took off in the mid-90's, roughly 20 years ago. So I was off by a few years, you're right.
(But I'd say the idea of free/libre software certainly predates even the GNU manifesto.)
What? When Linux was first released, the FSF had already written and released a ton of free software under the GPL, including Emacs (first release 1985), gcc (first release 1987), free versions of the unix command line utilities, etc.