I did this same thing once on my GNex. Only I was dumb enough to do it in the actual gallery app. I deleted ALL my pictures of my newborn son on my phone. Luckily, we used multiple cameras so I still had some pictures from other sources. This is why I use Dropbox (and Google+) to automagically backup my photos whenever I connect to wifi. That way, if it happens again, they're all backed up.
Sent from my Nexus 5 using xda app-developers app