The Next Web explains what we've known all along:

Continuing its love letter to the noble GIF, Google is rolling out a new feature which displays animated previews of Google+ videos. This follows the launch of Google+’s Auto Awesome GIF maker, GIF support in Google+ profile photos, and GIF image search.

According to Google, animated previews began rolling out on June 28. The company tells us this feature is only enabled for new videos uploaded to Google+, and that “at this time, these do not include YouTube videos.” This statement suggests, however, that YouTube videos may eventually be supported.

The previews are actually quite helpful, as they let users take in a summary of a video before committing to watching it in full.

Yes! YES!! Just as we've been saying for years!!!

And no YouTube support?! WHY NOT???

Btw, Google GIF search has been awesome since it rolled out in March:

https://plus.google.com/+google/posts/F1YjHaT7ZUk