I did not get my 50 gems for reaching top 500 at the Monster Feeding event that just ended this morning.Same thing happened more than an year ago,and i didnt recieve anything.The gold prizez for other events(lower positions) i did recieve,but never the gem rewards...

Send a ticket to support, they'll help you.

And this is how to do it
https://forums.socialpointgames.com/topic/30/how-to-report-game-problems-to-support