According to the Glock company's official story, that's correct.

The Glock 7 is a fictional weapon, it doesn't cost as much as John McClane said, and a gun barrel is still made of metal to deal with the explosive pressure of rapidly expanding gases.

There was also a composite gun used in 'In the Line of Fire', a Clint Eastwood movie from 1993.

It didn't originate in the Die Hard movie. There was a media fueled rumor about the Glock 17 containing so little metal that it wouldn't show up on metal detectors, and Die Hard 2 merely perpetuated that idea. However, none of it was ever true.

In the mid '80s there was a rumor about a Soviet all-plastic 3-shot gun, and there was a guy who patented the idea of an all-plastic gun, and then the media picked up on Glock making guns using plastic frames (but still more than 80% metal by weight). Then someone wrote a breathless story about how Libya was trying to buy Glocks, and this rapidly snowballed into a media fueled freakout about plastic guns and the Glock became a "terrorist special" and so forth.

Die Hard 2 I believe. That's where I first heard it too.